In this work, the reaction mechanism of photocatalytic oxidation of sacrificial ethanol during water-splitting process by titanium dioxide (TiO2) has been studied. The pure rutile TiO2 or mixed-phase structure titania (P25) was employed as the typical photocatalyst in ethanol oxidation. The as-obtained results showed that the formation of 2,3-butanediol over TiO2 in heterogeneous systems is mainly due to the photochemical reaction proceeded between acetaldehyde molecule and ethanol molecule instead of the direct coupling of α-hydroxyethyl radicals. This is different from the early work claimed that the fundamental process to produce 2,3-butanediol is based on the direct coupling of α-hydroxyethyl radicals generated by TiO2 oxidation. The photochemical reaction between acetaldehyde molecule and ethanol molecule to form 2,3-butanediol can also occur when the concentration of the solid catalyst was reduced to certain degree if using P25 as catalyst in heterogeneous model, and the selectivity of 2,3-butanediol would change from ca. 60% to 0% when enlarging the concentration of P25 step by step. However, the selectivity of 2,3-butanediol is relatively invariable when the concentration of catalyst was changed if using rutile as photocatalyst. We thought that the distinct diffusing behaviors for mobile ·OHf and surface bound ·OHs generated on different titania can explain the varied selectivity when the solid concentration of TiO2 changed. The generation and diffusion of ·OH from the surface of P25 (80% anatase) to bulk solution is a key process to inhibit the direct coupling of α-hydroxyethyl radicals to produce acetaldehyde or further overoxidation products, and the reaction zone of ·OHf depends on the concentration of P25. For the case of rutile TiO2 promoted reaction, the lack of mobile ·OHf on rutile TiO2 makes the photochemical reaction between acetaldehyde molecule and ethanol molecule more facile to occur in bulk solution since the surface bound ·OHs can only have chance to attack the surface adsorbed substrates. This may be an important reason to explain why the selectivity of 2,3-butanediol in ethanol oxidation was not influenced significantly by the variation of rutile TiO2 concentration. All the results regarding ethanol transformation during photocatalytic process achieved here cast some light on the mechanistic understanding of the reactions proceeded on the surface of solid catalyst in heterogeneous model and in the bulk solution when both catalytic step and photochemical step existed simultaneously.