Photolysis of CO2 at 185 nm using a Hg lamp in the presence of H-2 or H2O leads to the formation of CH4 (after an induction period) and a lesser amount of CO. Using H-2 gas as reducing agent, up to 40% conversion was achieved with almost complete selectivity to methane and energy consumption of 1.55 Gcal/mol. When CO2 irradiation is carried out in the gas phase in the presence of H2O, both reagents can undergo photolysis, and H-2 generation from water is a competing process accompanying the formation of CO and CH4 (CO2 conversion 0.67% at 14 h). For the irradiation of CO2 using H2O as reducing agent, basic solids (K+-exchanged Y zeolite or hydrotalcites) can increase CO2 conversion up to 2.21% with total selectivity toward CH4 and an energy consumption of 4.5 Gcal/mol. This result opens a door in the quest for efficient photocatalysts that could further increase the apparent quantum yield of the process, particularly at longer wavelengths.