Adsorption isotherms of alizarin red S on hydroxyapatite were supplemented with additional data to provide a simple model of adsorption. The model treats the adsorbing surface as a mosaic of unit cell a-c faces which have an area of 65 A2 each. The adsorbate of the model has molecular dimensions consistent with a 1:1 relationship between adsorbate and adsorption site. By comparing slurries of hydroxyapatite which differed by a factor of 4 in surface area of the solid, it was found that the isotherm based on edge-on adsorption was best simulated by the model. A method for compensating for differing degrees of surface coverage is described.