ACS citation style: formats, worked examples, and common errors

If you are writing a chemistry thesis, a lab report, or a manuscript for an ACS journal, your references will be inspected before anyone gets to your results. ACS style is compact and rule-dense: numbered citations, abbreviated journal titles, semicolons between authors, bold years, italic volume numbers. Get one convention wrong and the mistake repeats through every entry in your reference list.

The style belongs to the American Chemical Society, and the current authority is the ACS Guide to Scholarly Communication, the online successor to the printed ACS Style Guide. Unlike APA or Harvard, ACS does not commit to a single in-text format. It defines three, and the journal or department you are writing for picks one, so the first job is always to find out which variant you are expected to follow.

This guide covers the three in-text formats, the numbering rules, the reference list conventions that cause the most corrections, worked examples for the source types chemists actually cite, the errors reviewers flag most often, and how to generate correct ACS references automatically instead of hand-formatting them.

Who uses ACS citation style and when it is required

ACS style dominates chemistry and its neighbours: chemical engineering, materials science, biochemistry, chemical biology, and parts of pharmacology. If you are submitting to an ACS journal the style is mandatory, and most university chemistry departments require it, or a close local variant, for theses and dissertations. You will also meet it in coursework, because many departments grade lab reports against it precisely so that journal submission will not be your first attempt.

Requirements differ in the details. One journal wants superscript numbers, another wants italic numbers in parentheses, and a supervisor may have opinions about whether article titles belong in the reference list. Before you format anything, check the author guidelines for your target journal or the thesis handbook for your department, and note which choice they make on each point below.

ACS is one of several numeric citation families, and it is easy to confuse with Vancouver or IEEE at a glance. The tells are the semicolons between authors, the bold year, and the CASSI journal abbreviations. If a template you found online lacks those, it is not ACS.

The three ACS in-text citation formats

ACS defines three ways to point at a reference from your text. They are not interchangeable within one document: pick the one your journal or department specifies and use it everywhere.

• Superscript numbers, placed after punctuation: 'The intermediate is a radical.1,3' (the 1,3 is superscript). This is the most common choice in ACS journals.
• Italic numbers in parentheses on the line: 'The intermediate is a radical (1, 3).' The numbers are italicised to distinguish them from equation and compound numbers.
• Author-date: 'The intermediate is a radical (Garcia and Chen, 2019).' Rare in ACS journals but still defined by the guide.

How ACS reference numbering works

Numeric ACS citations follow two rules. References are numbered in the order they first appear in the text, and a source keeps its original number every time you cite it again. If reference 4 comes back in your conclusion, it is cited as 4, not renumbered.

Multiple citations at one point are combined: consecutive numbers become a range, so references 3, 4, 5 and 6 appear as 3-6, while non-consecutive numbers are separated by commas, as in 1,5,12. Citation numbers go after commas and full stops, not before them.

Author names can still appear in the sentence when the science needs them: 'Garcia and Chen showed that surface doping shifts the onset potential.7' The number does the citing; the names are prose. Text mentions use both surnames for two authors and the first surname with 'et al.' for three or more, while the reference list itself normally names every author.

The practical problem is revision. Delete a paragraph from your introduction and every number after it shifts, and any hand-typed citation in the text now points at the wrong entry. Doing this bookkeeping manually across a thesis chapter is where most ACS numbering errors are born, and it is the single best argument for generating references from software rather than typing them.

ACS reference list rules that trip people up

The reference list is where ACS is most particular. These are the conventions that generate the most corrections:

• Author names are inverted and separated by semicolons, with initials after the surname: Garcia, M. L.; Chen, H. Never use 'and' between authors.
• Journal titles are abbreviated according to CASSI, the CAS Source Index: J. Am. Chem. Soc.; Angew. Chem., Int. Ed.; J. Phys. Chem. C. Do not guess abbreviations; look them up.
• The publication year is set in bold and the volume number in italics, followed by the page range: 2019, 123, 4567-4578.
• Article titles are included in current ACS practice, set in title case and ending with a period. Older papers omitted them, so do not copy formats from decades-old bibliographies.
• Page ranges are written in full: 4567-4578, not 4567-78.
• Issue numbers are usually omitted unless each issue of the journal restarts at page 1.

Worked examples: journal article, book, chapter, thesis, and website in ACS format

A formatting note before the examples: plain text cannot show bold or italics, so remember that in your document the journal abbreviation and volume number are italic and the year is bold.

Journal article: Garcia, M. L.; Chen, H. Photocatalytic Degradation of Azo Dyes on Modified TiO2 Surfaces. J. Phys. Chem. C 2019, 123, 4567-4578.

Book: Hartwig, J. F. Organotransition Metal Chemistry: From Bonding to Catalysis; University Science Books: Sausalito, CA, 2010; pp 261-320. Note the semicolon before the publisher and the 'Publisher: City, Year' order, which is the reverse of most humanities styles.

Book chapter: Okafor, C. Singlet Oxygen Pathways in Polymer Degradation. In Advances in Polymer Photochemistry; Meyer, T., Ed.; Springer: Berlin, 2017; pp 101-134.

Thesis: Nguyen, T. Electrochemical CO2 Reduction on Copper Alloy Catalysts. Ph.D. Dissertation, University of Manchester, Manchester, U.K., 2021.

Website: National Institute of Standards and Technology. NIST Chemistry WebBook. URL (accessed 2026-03-02). Replace URL with the full address, and always include the accessed date, because web content changes.

Preprints and datasets follow the same logic: name the repository, such as ChemRxiv or Zenodo, give the identifier or DOI, and include the year. When a preprint is later published, cite the journal version instead.

Common ACS citation errors reviewers and examiners flag

These are the mistakes that come back circled in red:

• Commas or 'and' between author names instead of semicolons.
• Guessed journal abbreviations that do not match CASSI, or full journal titles left unabbreviated.
• Reference lists ordered alphabetically when the in-text format is numeric; numeric ACS lists follow order of first citation.
• A re-cited source given a new number instead of its original one.
• Truncated page ranges like 1234-56.
• Mixed in-text formats, superscripts in one chapter and parenthetical numbers in another, usually after merging files.
• References copied from another paper's bibliography without checking the details against the actual article.

Generate ACS citations automatically

The last error in that list deserves emphasis. Second-hand reference entries inherit second-hand mistakes, and examiners do check whether the volume and page numbers resolve to the paper you cite. So start from real metadata: if you have a DOI, run it through the free DOI lookup to pull the paper's actual details rather than retyping them from a PDF header, then format the reference with the free ACS citation generator.

For a thesis or manuscript, formatting one reference at a time does not solve the real problems: numbering drift, style consistency, and references that quietly point at the wrong paper. Inside CiteDash, the Reference Manager holds every citation as a database object that resolves to a real paper, never free text, and formats your bibliography in ACS or any of 12 supported styles, so a late switch from ACS to Vancouver is a setting, not a lost weekend.

When you compile with the Thesis Assembler, bibliographies are built with citeproc for DOCX, PDF, and LaTeX output, which means the numbering is regenerated on every compile and cannot drift out of sync with your text. That matters most at the end of a project, when a supervisor asks for one more revision cycle and every reference number would otherwise need re-checking by hand.

Check every ACS reference before you submit

Chemistry has an active retraction record, and citing a retracted paper in a thesis is an avoidable embarrassment. CiteDash shows retraction badges in your library and blocks retracted papers from citation, and you can screen any DOI with the free retraction checker.

If you use AI anywhere in your workflow, grounding matters more than formatting. CiteDash grounds AI claims in the full-text PDFs it holds, and its Fact Checker verifies each claim against the source before it reaches you, so the sentence and the citation attached to it are checked together, not just the reference format.

Formatting earns you no marks, but bad formatting costs them. Sort the conventions once, automate the repetition, and spend the reclaimed hours on the chemistry.

ACS style FAQs: DOIs, et al., and supporting information

Do you include DOIs? Increasingly yes, and always for articles published online ahead of print, where the DOI is the only stable locator. Placement follows the journal's guidelines; when in doubt, add it at the end of the reference.

When does 'et al.' appear? In text mentions with three or more authors. In the reference list, standard ACS practice is to name every author, though some journals truncate very long author lists, so follow the journal you are submitting to.

How do you cite supporting information? Cite the parent article. Supporting information travels with the paper it belongs to and does not get its own reference entry.

Can you cite a paper you have not read? You should not. If you must credit an idea you met second-hand, cite the source you actually read and make the relationship explicit in the sentence.

ACS style quick checklist

Before you hand the document over, run down this list:

• One in-text format used throughout, matching the journal or department requirement.
• References numbered by first appearance, with original numbers reused on re-citation.
• Semicolons between authors, initials after surnames.
• CASSI abbreviations for every journal title.
• Bold years, italic volumes, full page ranges.
• Accessed dates on every web source.
• Every reference checked against the real paper, and no retracted sources.