Olefins such as propylene and isobutylene are converted to the corresponding unsaturated nitriles, acrylonitrile, and methacrylonitrile, respectively, by reacting a mixture of the olefin, ammonia, and molecular oxygen-containing gas in the presence of a catalyst containing the oxides of bismuth, molybdenum, iron, nickel and magnesium, at least one element selected from the group comprising potassium and cesium, and optionally one element selected from the group comprising cobalt, phosphorus, manganese, tellurium, sodium, cerium, chromium, antimony and tungsten wherein the sum of cesium and potassium is at least 0.1 to 0.4.