Under stressed conditions, hydrolysis of the trifluoromethyl moiety of tolrestat (1) to the dicarboxylic acid analogue (2) is the major degradation pathway in solution; greater than C = S bond hydrolysis of the thioamide moiety with formation of the oxo analogue (3) is the major solid-state degradation pathway. Rotamerization and degradation reactions in solution occur simultaneously and follow pseudo first-order kinetics. No appreciable buffer effect on the degradation of tolrestat is observed. The pH-rate profile exhibits specific acid catalysis (kH) and neutral water catalysis (ko). When tolrestat in solution and solid state is exposed to fluorescent and UV light, degradation reactions generate similar products to those found in the thermal reaction. No oxygen effect on the degradation reaction is observed.