Artificial photosynthesis has recently drawn an increasing amount of attention due to the fact that it allows for direct solar-to-chemical energy conversion. However, one of the basic steps of this process, namely the reduction of CO₂ by H₂O to afford O₂ and CO₂ reduction products (CO₂RPs) such as HCOOH, CO, HCHO, CH₃OH, and CH₄, is very difficult to achieve. In contrast to the CO₂ reduction in plants and homogenous systems, the reduction of CO₂ to CO₂RPs over heterogeneous photocatalysts was challenged by the competing reduction of H⁺ to H₂. Unfortunately, most of the research performed so far has focused only on the reduction of CO₂, rather than the characterization of the H₂O oxidation and H₂ production. Moreover, the fact that the heterogeneous photocatalytic reduction of CO₂ into CO₂RPs by H₂O should satisfy several selectivity criteria has often been ignored. Herein, we propose three such evaluation criteria, namely (1) the origin of carbon in CO₂RPs (determined using isotopically labeled CO₂ (¹³CO₂)), (2) the relative amount of H₂ and CO₂RPs produced, and (3) the amount of O₂ produced by the oxidation of H₂O. If all these criteria are satisfied, i.e., the carbons of CO₂RPs originate from CO₂, the amount of H₂ produced is negligible, and a stoichiometric amount of O₂ is produced by the oxidation of H₂O, then CO₂ introduced into the gas phase is believed to be reduced by H₂O to CO₂RPs in the aqueous phase.