The role of the electrons and holes at the surface of semiconductor oxides (TiO 2 and WO 3 ) in heterogeneous photocatalysis has been investigated in aqueous media for the reactions involving the series: nitrobenzene, nitrosobenzene, phenylhydroxylamine, aniline, and the related compound, 4-nitrosophenol. Qualitative and quantitative evaluation of most intermediates and their time evolution suggest that the reductive pathways are important and even predominant under a variety of experimental conditions. This aspect is not only true at the beginning of the process or for the readily reducible structures, but also during the entire degradation process. Each compound of the series is converted to all the others, even though in widely different amounts. In the early part of the photocatalytic process, with nitrosobenzene and with phenylhydroxylamine, even in the presence of oxygen, the nitrogen substituent undergoes simultaneous oxidation and reduction at comparable rates, so that very little change in the mean oxidation state of the system is observed. This suggests that photogenerated electrons have a controlling role, particularly for some compounds in the early steps of the photocatalytic transformation. For 4-nitrosophenol and p-benzoquinone, in the early steps of degradation the reductive pathways represent the main route, even in the presence of oxygen. As a consequence, for some compounds the presence of an excess of oxygen in the reacting atmosphere decreases the degradation rate, instead of promoting it, as is commonly observed in photocatalysis. It is also remarkable that, for some compounds examined, the redox reactions at the nitrogen-containing substituent have a comparable or even more important role than the hydroxylation of the aromatic ring, which was the predominant degradation pathway for most of the other aromatic compounds.