A pulsed reaction technique was applied to discuss the effect of support on the activities and mechanisms in the CO2 reforming of methane over Ru catalyst. The reaction was carried out using a fixed bed reactor equipped with an on-line mass spectrometer. Four supports: La2O3, Y2O3 and ZrO2 which showed high activity and Al2O3, commonly used one in the reforming reaction, were compared when loaded with Ru. After feeding CO2 at 600 degrees C, we introduced a pulse of CH4 over Ru/La2O3 catalyst under Ar steady flow. We observed the response of CO which was generated from the reaction with CHx on the ruthenium and the Ru-O-x formed during CO2 treatment or during the reaction of Ru-CHx with adsorbed CO2 onto the La2O3. Over Ru/Al2O3 catalyst, however, very small response of CO was observed. A pulse of (CO2)-C-13 was introduced under CH4 steady flow over Ru/La2O3, Ru/Y2O3 and Ru/ZrO2 catalysts. Symmetrical (CO)-C-13 responses were observed, but a small response of (CO)-C-12 from (CHx)-C-12 continued to evolve after generation of (CO)-C-13 from (CO2)-C-13 ceased. The following reaction cycle is believed to occur in the CO2 reforming of methane on active supports: A part of metallic ruthenium reacted with CH4 to give Ru-CHx; simultaneously ruthenium metal could be oxidized with CO2 to give Ru-O-x and CO; and then, oxygen transfer from Ru-O-x to Ru-CHx took place to give CO and metallic ruthenium. Distinct temperature increases in the catalyst bed for La2O3, Y2O3 and ZrO2 supports were observed with the introduction of CO2 pulses under Ar flow. On the other hand, a very small increase in the temperature of the catalyst bed was observed on Al2O3 These results indicate that CO2 reforming of CH4 with ruthenium loaded catalysts was strongly assisted by the activation of CO2 adsorbed on the basic sites.