The 13C/12C isotopic ratios of samples of acetic acid of biogenic origin obtained by fermentation and of synthetic ones produced from petroleum were determined. The biogenic products had a higher 13C content than the synthetic ones. The origin of vinegar can be determined on this basis. Biogenic acetic acid possesses a lower "C content in the methyl than in the carboxyl group. This regularity was not observed for synthetic acetic acid. The ''C distribution within the molecule seems to depend on the type of synthetic process applied.