N,O-Dicarbethoxy-4-chlorobenzenesulfohydroxamate (1c) and O-carbethoxy-N-hydroxysaccharin (6), both potential carbethoxylating agents, inhibited yeast aldehyde dehydrogenase (AlDH) with IC50's of 24 and 56 microM, respectively. The esterase activity of the enzyme was commensurably inhibited. AlDH activity was only partially restored on incubation with mercaptoethanol (20 mM) for 1 h. On incubation with rat plasma, 1c liberated nitroxyl, a potent inhibitor of AlDH. Under the same conditions, nitroxyl generation from 6 was minimal, a result compatible with a previous observation that nitroxyl generation from N-hydroxysaccharin (7), the product of the hydrolysis of the carbethoxy group of 6, was minimal at physiological pH. Since chemical carbethoxylating agents represented by the O-carbethoxylated N-hydroxyphthalimide, 1-hydroxybenzotriazole, and N-hydroxysuccinimide (8, 9, and 10, respectively) likewise inhibited yeast AlDH, albeit with IC50's 1 order of magnitude higher, we postulate that 1c and 6 act as irreversible inhibitors of AlDH by carbethoxylating the active site of the enzyme.