Abstract: Catalysts of the formula I?A.sub.a B.sub.b O.sub.x !.sub.p ?C.sub.c D.sub.d Fe.sub.e Co.sub.f E.sub.i F.sub.j O.sub.y !.sub.q I,whereA is bismuth, tellurium, antimony, tin and/or copper,B is molybdenum and/or tungsten,C is an alkali metal, thallium and/or samarium,D is an alkaline earth metal, nickel, copper, cobalt, manganese, zinc, tin, cerium, chromium, cadmium, molybdenum, bismuth and/or mercury,E is phosphorus, arsenic, boron and/or antimony,F is a rare-earth metal, vanadium and/or uranium,a is from 0.01 to 8,b is from 0.1 to 30,c is from 0 to 4,d is from 0 to 20,e is from 0 to 20,f is from 0 to 20,i is from 0 to 6,j is from 0 to 15,x and y are numbers determined by the valency and frequency of the elements other than oxygen in I, and p and q are numbers whose ratio p/q is in the range from 0.001 to 0.