Abstract: In the preparation of methacrylonitrile by gas-phase ammoxidation of isobutylene or tertiary butanol, the reaction is carried out (A) (i) using a catalyst system consisting essentially of (1) 30-70% by weight of a silica catalyst carrier and (2) a catalyst material having a composition represented by the formula:A.sub.0.05-1.0 Mo.sub.12 Bi.sub.1-18 Fe.sub.2-10 Na.sub.0-3.6 P.sub.0-3 O.sub.xwherein A is at least one element selected from the group consisting of potassium, rubidium and cesium, and x is the number of oxygen atoms needed to satisfy the valence requirements of the other elements present, said catalyst system having a specific surface area of 2-10 m.sup.2 /g; (ii) at a temperature of 380.degree.-480.degree. C.; (iii) at a contact time of 1-10 seconds; and (B) while maintaining the oxygen concentration in the gaseous reaction mixture below about 1 percent by volume.