1. Two-different double derivatization techniques and two different g.l.c. systems were used to separate isomeric phenolic carboxylic acids in normal human urine.2. Carboxylic acids were converted into n-butyl esters and phenolic functions into trifluoroacetic acid esters.3. n-Butyl trifluoroacetoxybenzoates were separated by g.l.c. and detected by mass spectrometric single-ion monitoring.4. Trifluoroacetates were hydrolysed under mild conditions, and liberated phenolic groups were subjected to flash methylation.5. n-Butyl methoxybenzoates were separated by g.l.c. and detected by flame ionization.6. All derivatives were identified by comparison of retention times and mass spectra with those of authentic reference standards.7. The urine of a normal vegetarian volunteer was examined.8. The presence ofmeta- andpara-hydroxybenzoic acids, vanillic acid and isovanillic acid was confirmed by unambiguous techniques.