Abstract 1. 1. A study was made by a continuous-circulatory method of the properties of SbTeMoO catalysts in oxidation of isobutylene to methacrolein. 2. 2. X-ray studies of the phase composition and the dependence of catalytic properties of catalysts on Mo content indicate that the addition to an initial SbTeO catalyst of Mo oxide produces a chemical compound (or compounds) of Mo, Sb and Te oxides. The formation of a chemical compound (or compounds) and an increase in concentration considerably increases the specific catalytic activity of the initial catalyst. Catalysts with optimum properties with an atomic ratio of Sb : Te=4 : 1 should contain Mo in a proportion of up to 60 at·%. 3. 3. The order of formation of products of oxidation of isobutylene and the activation energies of these reactions were determine both with individual oxides and catalysts mixed with SbTeO and SbTeO with the addition of 60 at·% Mo.