Deuterated tyrosine, 4-hydroxyphenyllactic acid, 4-hydroxyphenylpropionic acid, 4-hydroxyphenylacetic acid and 4-hydroxybenzoic acid were incubated under anaerobic conditions with human faecal specimens for the in vitro study of their respective metabolisms. After 1 week, aromatic acids and phenols were extracted and analyzed by gas chromatography-mass spectrometry. [3',5'-2H2]Tyrosine produced 4-hydroxyphenyllactic acid, 4-hydroxyphenylpropionic acid and 4-hydroxyphenylacetic acid; [3',5'-2H2]-4'-hydroxyphenyllactic acid produced 4-hydroxyphenylpropionic acid, 3-hydroxyphenylpropionic acid, 4-hydroxyphenylacetic acid and phenylproionic acid; [3',5'-2H2]-4'-hydroxyphenyl-propionic acid produced 3-hydroxyphenylpropionic acid and phenylpropionic acid; [3',5',2,2-2H4]-4'-Hydroxyphenylacetic acid produced p-cresol; and [3',5'-2H2]-4'-hydroxybenzoic acid produced phenol. Thus the intestinal flora showed activities for decarboxylation leading to phenol and p-cresol, dehydroxylation leading to phenylpropionic acid and rearrangement leading to 3-hydroxyphenylpropionic acid. Rentention of both deuterium labels was observed in the rearrangement reaction.