Ti-SBA-15 was exposed to illumination in the presence of different gas mixtures containing CO or CO2, and H2O or H2, in order to clarify the route to hydrocarbon formation in photocatalytic CO2 reduction over this photocatalyst. A mixture of CO and H2O led to the highest quantities of CH4, C2H4, and C2H6 after 7 h of reaction, whereas a mixture of CO2 and H2 lead to the lowest production rate of these products. H2O has been identified as more efficient in activation of CO and CO2 than H2. CH3OH was not detected as significant product, and when fed to the catalyst, did not yield extensive product formation. Formaldehyde was found very reactive over the catalytic system, yielding a product distribution (C1–C2) of similar nature as obtained by CO activation. Finally, backward reactions, i.e., oxidation of hydrocarbon products into CO or CO2, were found significant. Based on the experimental activity profiles, results indicated above, and available literature, a mechanism for photocatalytic CO2 reduction is proposed involving the formation of CO in the initial stages, followed by consecutive formation of formaldehyde, which converts to CH4, C2H4, and C2H6, presumably by reaction with photo-activated H2O (OH radicals).