The kinetics and mechanism of oxidation of an important xenobiotic, thionicotinamide (TNA), using peracetic acid in slightly acidic media were studied by spectrophotometric techniques. The reaction is characterized by a very rapid initial oxidation of the sulfur atom of the thioamide group to the S-oxide, followed by a much slower decomposition of the S-oxide to form additional oxidation products, mainly the thionicotinamide sulfinic acid. In excess thionicotinantide, the stoichiometry of the reaction was determined to be CH3CO3H + (C5H5N)C(=S)NH2 --> (C5H5N)C=NH)SOH + CH3COOH, whereas in excess peracetic acid the stoichiometry was 2:1, 2CH(3)CO(3)H + (C5H5N)C=S)NH2 + H2O --> (C5H5N)C(=NH)SO2H + 2CH(3)COOH + 2H(+). The sulfoxide is quite stable, but there was no experimental evidence for the existence of any stable sulfone-sulfonic acid intermediates. Results show that the sulfur atom in thionicotinamide is the reactive center, undergoing a stepwise addition of oxygen to form successively the sulfenic acid and the sulfinic acid. There appeared to be no further oxidation past the sulfinic acid and no formation of sulfate was observed. A bimolecular rate constant of (1.1 +/- 0.3) x 10(3) M-1 s(-1) was evaluated for initial rapid formation of the S-oxide, and an upper limit rate constant of 0.10 +/- 0.02 M-1 s(-1) was evaluated for the slower decomposition of the S-oxide.