The constant capacitance model was used to describe phosphate adsorption on hematite, kaolinite, and a kaolinite-hematite system (k-h), The model assumes a ligand exchange mechanism and considers the charge on both adsorbate and adsorbent, The model is shown to provide a quantitative description of phosphate adsorption on these, including the effect of varying pH values, The computer program Ma-Za 2, a program that fits equilibrium constants to experimental data using an optimization technique, was used to obtain optimal values for the anion surface complexation constants on hematite, kaolinite, and a kaolinite-hematite system, while the PC program Ma-Za 1 in Q-Basic language was used for the application of the constant capacitance model. The model represented adsorption of phosphate anions well over the entire pH range studied (3.8-9.0), The main advantage of the model is its ability to represent changes in anion adsorption occurring with changes in pH. Extension of the model to describe phosphate adsorption in a mixed system, such as the kaolinite-hematite system, using the surface protonation-dissociation constant of hematite was qualitatively successful. In mixed system the model reproduced the shape of the adsorption isotherms well over the pH range 3.8-9.0. However, phosphate adsorption was overestimated. The hematite and the kaolinite-hematite system were synthesized and identified by X-ray, NMR, and FT-IR spectroscopy.