TiO2 is an active material for photocatalytic CO2 reduction. Its performance is improved by the addition of a metal or metal oxide as a cocatalyst. The enhanced electron-trapping capability is widely attributed to the function of these cocatalysts, but their precise roles are not fully understood. Here, we report how Pt and Co cocatalysts boost the formation of H2 and CH4 during photocatalytic CO2 reduction. More specifically, we used in situ diffuse reflectance infrared Fourier transform spectroscopy and multivariate spectral analysis to identify (in)active surface intermediates. Surface formates were identified as reactive intermediates common to all TiO2-based catalysts. The catalytic activity and product selectivity are determined by the unique function of the cocatalyst, which distinctly interacts with the TiO2 surface to produce and decompose formates to H2 or CH4. The evolution of the surface species also clarifies the transient nature of photocatalytic activities and how the TiO2 surface and cocatalysts are deactivated under photocatalytic conditions.