Various metal oxides were investigated using laser pulse photo-electromotive force measurements. This method is possible if the band gap of these oxides is smaller than the laser radiation energy. The sign of the photovoltage and its decay rate depend on the specific surface potential, which is influenced by adsorption layers. The adsorption of acids and bases on ZnO, TiO2, alpha-Fe2O3 and gamma-Fe2O3 microcrystals shows the dependence of the photovoltage on the type of space charge layer on the surface of the oxides. Oxides with a depletion layer exhibit a rising photovoltage with rising pH, whilst oxides with an accumulation layer at the surface exhibit the opposite behaviour.