Aldehyde dehydrogenase can catalyze the hydrolysis of esters such as p-nitrophenyl acetate as well as oxidize aldehydes to acids. It has not been proven unequivocally that the two reactions occur at the same active site. In the accompanying paper (Tu, G. C., and Weiner, H. (1988) J. Biol. Chem. 263, 1212-1217) evidence was presented which showed that cysteine at position 49 was at the active site for the dehydrogenase reaction. Evidence also was presented which showed that cysteine located at position 162 was susceptible to modification by N-ethylmaleimide. It was shown here that the two activities of the enzyme can be differently protected from inactivation by substrate analogs. Furthermore, aldehydes were found to be poor inhibitors against the esterase reaction while ester was a good inhibitor against the dehydrogenase reaction. In addition, it was possible to modify cysteine 49 with N-ethylmaleimide but not find inhibition of the esterase reactivity until cysteine 162 was modified. It appears that horse liver aldehyde dehydrogenase has two separate active sites per subunit. The data fit a model where ester can be hydrolyzed at both sites but that aldehyde oxidation occurred only at position 49.