1,2-Propanediol oxidoreductase, which reduces the L-lactaldehyde formed in the fermentation of L-fucose or L-rhamnose to L-1,2-propanediol in E. coli, was inactivated by a component of E. coli cell extracts in the presence of oxygen. Pure propanediol oxidoreductase preparations were shown to be inactivated in vitro by aerobic incubations in the presence of Fe3+ and ascorbate. The Fe3+ ascorbate-mediated inactivation reaction was inhibited by catalase, although not by superoxide dismutase. Under anaerobic conditions, the presence of H2O2 strongly inactivated the enzyme. Propanediol oxidoreductase was rapidly degraded in the presence of oxygen, while the native enzyme displayed high stability as long as no oxygen was present.