Nanocrystalline TiO2 (nc-TiO2) was modified by a simple post treatment with monometallic sodium orthophosphate solution. It is shown that the surface modification with an appropriate amount of phosphate obviously enhances the surface photovoltage responses of nc-TiO2 in the presence of O2, clearly indicating that the separation of photogenerated charges is greatly improved by promoting the photoelectrons captured by the adsorbed O2. This is well responsible for its much high photocatalytic activity for degrading representative gas-phase acetaldehyde, liquid-phase phenol and rhodamine B of phosphate-modified nc-TiO2, compared with the unmodified nc-TiO2. Moreover, it is demonstrated that the amount of O2 adsorbed on the surfaces of nc-TiO2 is greatly increased after phosphate modification based on the O2 temperature programmed desorption curves, which is attributed to the substitution of -Ti-OH with -Ti-O-P-OH. It is suggested for the first time that the phosphate modification favors the O2 adsorbed on TiO2 so as to further promote the photogenerated electrons captured. This work would provide feasible routes to further improve the photocatalytic performance for degrading pollutants of oxide-based semiconductors.