Using a cell designed to study the electronic interactions of gases with illuminated powders, the photoconductance, σ, of a series of oxides (TiO2, ZrO2, V2O5, ZnO, SnO2, Sb2O4, CeO2 and WO3) has been measured at equilibrium under various oxygen pressures P O2 (from 7 × 102 down to 0.7 Pa) at room temperature in an attempt to obtain information on the nature of negative oxygen species adsorbed on these illuminated surfaces. For the majority of the samples, the conductance of the non-illuminated part was negligible compared with that of the illuminated layer. In a log–log plot, the slopes of σ against P O2 were very close to –1, –½ or 0. A slope of –1 (found only for TiO2 on the higher-pressure side) and a slope of –½(observed for ZrO2, ZnO and, for part of the pressure range examined, for TiO2, Sb2O4 and CeO2) were interpreted as indicating that O–2 and O– species, respectively, controlled the equilibrium between the free electrons and the gas. SnO2 and WO3 were characterized by a poor relative response to illumination, so that σ variations with P O2 were masked; consequently the measurements failed in determining with certainty the type of adsorbed oxygen species, although the σ sensitivity to initial O2 admission showed the occurrence of ionosorption. The V2O5 sample did not behave as a photosemiconductor. This is consistent with the photocatalytic activities of these samples in propene oxidation (very small for SnO2 and WO3, nil for V2O5). However, no simple correlation exists between the photoconductance variations thus measured and the photocatalytic properties, which shows that the surface densities of electrons and of ionosorbed oxygen species are not the only factors to consider for oxidation reactions.