Abstract: A system for detecting carbon monoxide, even in the presence of methane, using a catalytic sensor material having a perovskite crystal structure and a compositionA.sub.1-n B.sub.n CO.sub.3-mwhere A is a rare earth metal; B is an alkaline metal; C is a metal from the group consisting of cobalt, vanadium and maganese; O is oxygen; n is less than 0.1; and m is not greater than 0.5, which material is substantially free of oxides of A, B and C. The method of making the crystalline material involves firing appropriate amounts of mixed salts of A, B and C until substantially converted to the perovskite structure, and then leaching with a caustic to remove the undesired oxides.