The mechanism of the partial oxidation of CH4 was studied by a transient response analysis of a broadened pulse combined with either a step change or a sharp isotopic pulse. Over the unreduced NiO/SiO2 catalyst, the reaction between CH4 and Oz occurs via the Eley-Rideal mechanism, CH4 in the gas phase (or weakly adsorbed) reacting with O-2 in the adsorbed state. In contrast, over the reduced NiO/SiO2 catalyst, the reaction takes place by a Langmuir-Hinshelwood mechanism, CH4 and O-2 reacting in the adsorbed states. In addition, isotopic pulses of O-18(2) revealed that over the reduced catalyst lattice oxygen is formed and reduced by the carbon species; hence, on the reduced catalyst a dynamic redox process occurs.