A broadly applicable theoretical and experimental framework was developed for understanding the photocatalytic mechanism of semiconductors. Using this framework, we found that it is possible to predict the type and reactivity of reactive oxygen species and electrons produced during photoexcitation of semiconductors by comparing the band edge energies of semiconductors with the redox potentials of relevant species. In addition, we could experimentally verify these predictions using electron spin resonance spectroscopy (ESR) with spin trapping and spin labeling techniques. We selected four types of metal sulfides (CdS, ZnS, In2S3, and Bi2S3) to elucidate the applicability of this model system. Using ESR technique, we found that these four sulfides are significantly different in the types of produced reactive oxygen species. When irradiated, ZnS can generate superoxide (O-2(-center dot)), hydroxyl radicals ((OH)-O-center dot), and singlet oxygen (O-1(2)); CdS and In2S3 can produce O-2(-center dot), while irradiation of Bi2S3 generates none of these reactive oxygen species. These results are correlated with the photocatalytic oxidation and reduction activities of metal sulfide structures. (C) 2014 Elsevier Inc. All rights reserved.