The selective oxidation of methane catalyzed by heteropoly compounds having the formulas MxCs2.5H0.5-2x+yPVyMo12-yO40 (M = pd(2+), Rh2+, Ru2+, Pt2+, Mn2+, Hg2+, Fe3+ Co2+, Cu2+; X = 0-3, y = 0-3) was investigated. It was demonstrated that addition of Pd and incorporation of V had a strong influence on oxidation with a H-2-O-2 gas mixture and that Pd0.08Cs2.5H0.34PVMo11O40 showed the highest yield of formic acid. The reaction proceeded at temperatures as low as 423-593 K. Moreover, the reaction rate reached 1.2 x 10(-4) mol h(-1) g(-1) at 573 K; this value is about 300 times higher than that with FePO4 catalyst. Addition of steam promoted the production of formic acid, and the yield reached a maximum at a partial pressure of steam of 9.1 kPa. The coexistence of H-2 and O-2 was indispensable for the selective oxidation of methane. It is suggested that an active oxygen species is formed by the reaction of H-2 with O-2 catalyzed by Pd and acidic sites of supports. Pressure dependencies were expressed by -dP(CH4)/dt = kP(H2)(1.0)P(O2)(1.0)P(CH4)(1.0), and is consistent with the idea that the reaction of an active species formed from H-2 and O-2 with CH4 is rate determining. (C) 2001 Academic Press.