An isotopic pulse is combined with a gas chromatography-mass spectrum to investigate the mechanism of the partial oxidation of methane. For a pulse reaction of CH4/CD4/O-2 (1/1/1) at 600 degrees C over a reduced NiO/ SiO2 catalyst, besides CH4 and CD4, large amounts of CHD3, CH2D2, and CH3D were detected in the exit gas. This indicates that CH, CH2, and CH3 species are formed during the reaction, i.e., that CH4 is activated via its dissociation before its oxidation. The amount of methane involved in the isotopic exchange reaction was larger than that converted to CO and CO2. Consequently, the dissociation of methane is not rate determining. Over an unreduced NiO/SiO2 catalyst, CH4 and CD4, without any CHxDy (x + y = 4), were detected in the exit gas, indicating that methane reacts directly with oxygen without its predissociation. For the reaction of CH4/CD4(1/1) with the lattice oxygen of the catalyst, CH4 and CD4, without any CHxDy (x + y = 4), were detected in the exit gas, indicating that methane reacts directly with the lattice oxygen without predissociation. Moreover, the CH4/O-16(2) reaction over an unreduced NiO18 catalyst revealed that the lattice oxygen of the catalyst participates in the reaction of CH4/O-2.