The online version of the ACS Style Guide can be found through the ACS Publications portal. Click on the link for "Publications" at the top right of the page and navigate to the "ACS Guide to Scholarly Communications."
ACS stands for the American Chemical Society. It is the citation style most commonly used in the field of chemistry.
The most current version (3rd edition) of The ACS Style Guide: Effective Communication of Scientific Information was published in 2006.