Systematic series of double perovskites containing W and Mo, A(2)(II)B(II)B(VI)O(6) (A(II) = Ba, Sr, Ca; B-II = Mg, Ca, Co, Ni, Cu, Cd; B-VI = W, Mo, W1-xMox) were synthesized, and their catalytic activities for NO-C3H8-O-2 reaction were investigated. All the oxides crystallized in the ordered double perovskite structure with the rock-salt arrangement of B-II and B-IV cations, and the unit cell symmetry was almost exclusively determined by A(II) cations. The double perovskites catalyzed the nonselective reduction of NO into N2O and occasionally N-2 specifically at 600 degrees C in the gaseous mixture of NO (0.44%), C3H8 (0.29%) and O-2 (4.4%). The reduction activity of NO into N2O and N-2 depended on the sort of A(II), B-II, and B-VI cations, but the effect of B-II cations was the most remarkable. The NO reduction activity increased monotonically with an increase in the enthalpy of formation of (BO)-O-II (-Delta H-f(o)), suggesting that the strength of B-II-O bond or the redox property of B-II ions is of primary importance for the NO reduction activity in the NO-C3H8-O-2 reaction.