The activity of molybdenum oxide supported on silica gel (MoO3/SiO2) for the photocatalytic oxidation of cyclohexane has been investigated. A comparison of results with those obtained in a previous investigation with V2O5/SiO2 indicates a similar behaviour, namely recrystallization involving photocatalytically active isolated species on the surface during the reaction. This has been confirmed by diffuse reflectance spectroscopic analysis of the catalysts before and after the reaction. From the two series of experimental results it can be concluded that two kinds of surface species exist: (1) Mo+6 cations in tetrahedral coordination, isolated and fixed on the surface, are active in photocatalysis; (2) Mo+6 cations in octahedral coordination, under the form of polymeric surface species, able to migrate in the conditions of the photocatalytic reaction. This migration leads to the fixation and agglomeration of these octahedral species and to the possible creation of Mo—O—Mo bridges between a tetrahedral and an octahedral species. In that case, the majority of the tetrahedral species does not change its coordination but loses its photocatalytic activity.