The influence of aqueous vapor on the activity of PdCuCaNaY catalyst with contents of cations of Ca²⁺ – 45, Pd²⁺ – 1.5 and Cu²⁺ – 6%, from the weight of NaY zeolite, in the reaction of n-butene gas-phase oxidation methyl­ethylketone has been studied. It has been established that preliminary treatment of catalyst by aqueous vapor strongly influences on its activity and selectivity in the n-butene oxidation reaction. It was determined that catalyst adsorbed by water with C_H2O=20% from zeolite weight displays high stationary activity and selectivity in the formation of methylethylketone. The stage scheme of mechanism of reaction in which water molecule takes direct participation in me­thylethylketone formation has been suggested