Copper- and vanadium-catalyzed oxidation of methane into methyl trifluoroacetate and formic acid with in situ generated H2O2 from H-2 and O-2 over Pd/C was investigated in trifluoroacetic acid (TFA) and/or trifluoroacetic anhydride (TFAA) as a solvent. Metallic palladium was responsible for the in situ generation of hydrogen peroxide from dihydrogen and dioxygen. Divalent copper hydroperoxide and monoperoxomonovanadate, VO(O-2)(+), seemed to be related to C-H bond activation. The radical chain reaction did not occur in either copper- or vanadium-catalyzed methane oxidation. However, methyl radical appeared to be produced in vanadium system but not in copper system. (C) 2003 Elsevier Science B.V. All rights reserved.