The contribution of a redox mechanism involving lattice oxygen in the catalytic combustion of methane over PdO/ZrO2 catalysts, prepared from amorphous Pd-Zr alloys, has been studied by means of gas pulse methods, including a novel technique "pulse thermal analysis", and using labeled catalysts containing (PdO)-O-18. Special emphasis was devoted to the influence of the isotope exchange (scrambling) of reactants and products, especially O-2 and CO2, with the catalyst on the quantity of O-18-containing reaction products. Substantial amounts of (H2O)-O-18 and (COO)-O-18-O-16 were detected during pulses of a reactant mixture consisting of methane and O-16(2) in a ratio 1:4 at 300 and 500 degrees C. The effect of the oxygen exchange of molecular oxygen with the solid phase proved to be negligible due to its low extent. At 300 degrees C, at least 20% of the CO2 formed originated from the redox mechanism involving lattice oxygen. At 500 degrees C, oxygen exchange of CO2 with the catalyst became predominant and precluded determining reliably the proportion of CO2 formed by the redox process. The results indicate that a substantial part of methane is oxidized via a redox process. Consequently, this reaction has to be taken into account when interpreting the catalytic behavior of palladium-based catalysts and explaining the structure-activity relations previously observed.