The specific adsorption of radiolabeled phosphate ions from perchlorate supporting electrolyte onto gamma-Al2O3 and hematite powder has been investigated. The pH dependence of the adsorption of phosphate species was compared with that of sulfate ions. It was demonstrated that in contrast to the behavior of sulfate ions the pH dependence of phosphate ions goes through a maximum. On the other hand, it was found that the reversible adsorption of phosphate ions is accompanied by the formation of strongly chemisorbed species. Results obtained from a study of the competitive adsorption of sulfate and phosphate ions at various pH values are reported and interpreted. An attempt is made to correlate the experimental findings with the models for anion adsorption reported in the literature.