Kinetics of the oxidation of γ-aminobutyraldehyde, catalyzed by NAD^+-γ-aminobutyraldehyde dehydrogenase purified from axes of soybean (Glvcine mox), were analyzed by steady state initial velocity and product and dead-end inhibition studies. In initial velocity studies with NAD^+ and γ-aminobutyraldehyde, Families of double reciprocal plots are linear and intersect at the left of the 1/v axis. NADH behaves as a mixed inhibitor against NAD^+ and γ-aminobutyrnldehyde. γ-aminobutyric acid does not inhibit the enzyme, even at concentrations as high as 50 mM. AMP is a mixed inhibitor against NAD^+ and γ-aminobutyraldehyde. Since these data are consistent with an Iso Ordered Bi-Bi steady state mechanism, the data fit a model where NAD^+ binds first while NADH dissociates last. Inactivation of catalytic activity was observed when the enzyme was treated with the thiol group-directed reagent NEM. The initial reaction of this enzyme followed pseudo-first order kinetics and the value for the second order rate constant of inactivation was 296.3 M^(-1)·min^(-1). The apparent order of reaction(N) was 0.77. Modification data suggest that two of four cysteine residues are associated with, or involved in, enzyme activity, and fluorescence spectra also support this interpretation.