AbstractNADP+‐dependent aldehyde dehydrogenase from the cytosolic fraction of the alkane‐degradating ‘Acetobacter rancens’ CCM 1774 was purified 112‐fold (specific activity of 112 nkat mg−1protein). After polyacrylamide gel electrophoresis of the purified enzyme only one band was visible. The relative molecular weight was estimated to be 82,000 by both gel filtration and disc gel electrophoresis. The substrate specifity of the purified enzyme within the straight chain aliphatic aldehyde series is confined to acetaldehyde and propionaldehyde. Butyraldehyde and formaldehyde are considerably less converted. NADP+alone served as electron acceptor. Mg2+stimulated the reaction velocity in a strong manner by ‘non‐competitive’ activation. The estimation of kinetic parameters and inhibition experiments of the irreversible oxidation of ethanal indicate a random kinetic mechanism at optimal aldehyde concentrations and saturating NADP+concentrations at optimal pH 8.5 which, however, trends to become ordered with decreasing pH values. Properties described exclude the participation of the enzyme in degradation ofn‐alkanes. One physiological function of this constitutive aldehyde dehydrogenase may be the intracellular detoxification of short chain aldehydes by conversion to corres