A catalyst composition comprising molybdenum, vanadium, antimony niobium, at least one element select from the group consisting of titanium, tin, germanium, zirconium, and hafnium, and at least one lanthanide selected from the group consisting of lanthanum, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium; with the proviso that catalyst contains germanium (in the absence of at last one of titanium, tin, zirconium, hafnium) only in combination with neodymium and/or praseodymium and no other lanthanides. Such catalyst compositions are effective for the gas-phase conversion of propane to acrylonitrile and isobutane to methacrylonitrile (via ammoxidation).