In this paper, a transient response analysis is developed by combining a broadened pulse with a step change or an isotopic pulse. The combined method provides information about the activation of the catalyst and the catalytic reaction mechanism. The method is applied to the CO2 reforming of methane over NiO/SiO2 at 800 degrees C. The response to the broadened CH4/CO2 (1:1) pulse over fresh NiO/SiO2 exhibited an induction time during which no reaction between CH4 and CO2 occurred, indicating that NiO/SiO2 had no activity. As the NiO was gradually reduced by CH4, the reforming reaction took place, indicating that Ni-0 is the active site for this reaction. During the transient response to a step change to He in the broadened pulse of CH4/CO2 over a reduced NiO/SiO2 catalyst at 800 degrees C, a long CO tail was observed; however, no such tail was noted in the response after a step change to He in the broadened pure CO pulse at 800 degrees C. This indicates that the CO desorption is rapid and that the surface reaction between C and O species constitutes the rate-determining step of the reforming reaction. The transient response to a sharp O-18(2) pulse introduced into the broadened CH4/(CO2)-O-16 pulse indicated that, over a reduced NiO/SiO2 catalyst, O-18(2) replaced some (CO2)-O-16 in the reaction with CH4 to generate CO18 and that some Ni-0 was oxidized to (NiO)-O-18. The long tail of CO18 indicates that carbon species reduce this NiO18 to Ni-0.