Applications of photocatalytic CO2 reduction are limited by poor overall efficiency and slow reaction rates. In addition to such intrinsic material’s limitations, external influences decrease observed reaction rates still further. This work aims to quantify those external factors in a batch photocatalytic gas-phase reactor for CO2 reduction on P25 (TiO2), approaching to the intrinsic activity of the material under standard conditions. The proposed methodology addressed optical aspects of the reactor design to decouple the influence of light in observed reaction rates; as a thin film with an optical effectiveness of 67%, P25 showed a yield of 14.5 [μmoles-C-products g-1 h-1], which is at least 20 times higher compared to previous references. A transport model demonstrated the small influence of diffusion based on a dimensional analysis and perturbation expansion. As a final remark, reaction mechanisms were analyzed using Microkinetics Analysis (MKA), whose qualitative predictions explained reaction trends not captured by coarse-grain models.