The mechanism of CO2/CH4 reforming over Ni-La2O3/5A has been studied. The results of the CO2-pulsing experiments indicated that the amount of CO2 converted was roughly proportional to the amount of H present on the catalyst, implying that CO2 activation could be H-assisted. Pulsing CH4 onto a H-2-reduced sample and a similar sample pretreated with CO2, we found that CH4 conversion was higher in the latter case. Hence, the idea of oxygen-assisted CH4 dissociation is plausible. The fact that the amount of CO produced in 10 pulses of CO2/CH4 was larger than that produced in 5 pulses of CO2 followed by 5 pulses of CH4, indicated that CO2 and CH4 could activate each other synergistically. In the chemical trapping experiments, following the introduction of CD3I onto a Ni-La2O3/5A sample pretreated with CH4/CO2, we observed CD3COOH, CD3CHO, and CD3OCD3. In the in situ DRIFT experiments, IR bands attributable to formate and formyl were observed under working conditions. These results indicate that formate and formyl are intermediates for syngas generation in CO2/CH4 reforming, and active O is generated in the breaking of a C-O bond. Based on these results, we suggest that during CO2/CH4 reforming, CO2 activation is H-promoted and surface O species generated in CO2 dissociation reacts with CHx to give CO. A reaction scheme has been proposed.