The systematic clarification of the photocatalytic process in different kinds of systems is significantly instructive to recognize the photocatalytic mechanism and design novel efficient photocatalysts. The types of the active species (such as (OH)-O-center dot and O-2(center dot-)) generated in the photocatalytic process are complicated and their roles playing in the photocatalysis remain obscure and controversial. Aimed at these problems, the active species in the degradation process of methyl orange (MO) were investigated by using TiO2 and ZnO photocatalysts. Various surface analysis techniques (NMR, ESR, photoluminescence technique, etc.) were used to study their mechanisms in the degradation of MO under UV light irradiation. As a result, it was found that in TiO2 system, (OH)-O-center dot was generated by photo-generated holes; while in ZnO system, (OH)-O-center dot was generated by photo-generated holes and O-2(center dot-). The larger numbers of the active species and their incessant sources in ZnO system contributed to the better activity. The conclusions were first found in the investigation of the active species between the two catalysts and would provide certain theoretical guidance in seeking the new pathway to increase the concentration of the active species and the photocatalytic activity. (C) 2013 Elsevier B.V. All rights reserved.