Diazomethyl beta-D-galactopyranosyl ketone (1) has been proven to be a mechanism-based, irreversible (suicide-substrate) inactivator of Aspergillus oryzae beta-D-galactosidase, but not an inactivator of E. coli lacZ beta-D-galactosidase. Compound 1 is stable in buffers of normal physiological pH. It is decomposed by H+, but not by nucleophiles. Inactivation of A. oryzae beta-D-galactopyranosyl ketone (2) nor diazomethyl alpha-D-galactopyranosyl ketone inactivated the enzyme and therefore inactivation is stereospecific, excess inhibitor could be separated from inactive enzyme without regain of activity and therefore it is bound irreversibly, and a second pulse of enzyme is inactivated at the same rate as enzyme inactivated to 95% activity by the first pulse. Diazomethyl beta-D-glucopyranosyl ketone (2) inhibited sweet almond beta-D-glucosidase.