The influence of oxygen in the feed on the very fast deactivation of a Pt/AI 2O 3 catalyst during gas-phase hydrogenation of the unsaturated aldehyde, 2-ethyl-hexenal, was investigated (50–160°C, P = 1 atm). The addition of oxygen (<0.5%) prevented deactivation and the products obtained were saturated aldehyde, saturated alcohol, deoxygenated aldehyde, and in a side reaction CO 2 and heptane. In the absence of oxygen, the simultaneous H 2D 2 exchange reaction was inhibited, and the products were saturated aldehyde, saturated alcohol, and in a side reaction CO and heptane. A tested Rh/Al 2O 3 catalyst was deactivated quickly like Pt/Al 2O 3 and the addition of oxygen restored the activity, while the tested Pd/Al 2O 3 and Ru/Al 2O 3 catalysts were not found to deactivate quickly and the addition of oxygen did not increase the activity. The fast deactivation on Pt/Al 2O 3 and Rh/Al 2O 3 might be explained by a side reaction producing CO ( RCHO → RH + CO) and the reactivating effect of oxygen addition might be explained by CO 2 produced instead of CO in the side reaction.