The extent and effects of leaching of amorphous microporous and mesoporous V- and Ti-containing silica catalysts during selective oxidation reactions with t-butyl hydroperoxide and hydrogen peroxide have been investigated under aqueous and water free reaction conditions. Nanoparticles of catalyst were found to obscure the analysis of the nature of the reactions. These particles, present in all experiments, cannot be removed by filtration or centrifuging and falsely contribute to apparent leaching. With the V-containing materials leaching leads to homogeneous V-species, which at least contribute, in all cases studied, to the overall reactivity. True leaching contributions are dependent on solvent, substrate and oxidant, and therefore, cannot be generalized. With the Ti-containing catalysts leaching is strongly dependent on reaction conditions.