The reaction mechanism for the selective oxidation of methane with hydrogen peroxide was investigated for a H4PV1Mo11O40 catalyst precursor, which has been reported to be the most active among various Keggin-type heteropolyacids and vanadium complexes in trifluoroacetic acid anhydride. The conversion vs selectivity relationships, the comparison of reactivities of products with methane, and kinetic results show that the first step, selective oxidation of methane into methanol or methyltrifluoroacetate, is rate-determining. The facts that the oxidation was much suppressed by the addition of a radical scavenger, that chlorocyclohexane was formed by the oxidation of cyclohexane in the presence of carbon tetrachloride, and that the epoxidation of cis-stilbene proceeded without retaining the stereochemistry show that the reaction includes a radical path. UV-vis data revealed that monoperoxomonovanadate is an active species, which would promote the selective oxidation of methane into methanol or methyltrifluoroacetate.