Sacrificial reagents (SRs) are extensively used in a variety of photocatalysis systems for hydrogen evolution from water decomposition. It was observed in this work that these SRs, such as Na2SO3, Na2S, KI, EDTA-Na, and CH3OH have the abilities to reduce H2O to H-2, and that AgNO3 and Fe(NO3)(3) have the abilities to oxidize H2O to O-2, even without the presence of photocatalysts. The water reduction and oxidation is attributable to the photochemical property of SRs acting as electron donors (EDs) and as electron scavengers (ESs), respectively, under irradiation. The influence of photochemical reactions of SRs on photocatalytic evolution of H-2 and O-2 could be avoided by using a 300 nm cutoff filter. Comparing the amounts of H-2 or O-2 evolved from the SR aqueous solution without Pt/TiO2 to those with Pt/TiO2, CH3OH and Fe(NO3)(3) were the most excellent ED and ES, respectively, for photocatalytic reduction and oxidation of water. It was proposed that the photochemistry between SRs and H2O should be considered when evaluating the activity of photocatalysts for photocatalytic reduction and oxidation of water into H-2 and O-2.