Sorption of cadmium ions from aqueous solutions on calcium hydroxyapatite was studied as a function of pH. The concentration of cadmium, [Cd](s), in the solid increases with pH. The total mean concentration of cations of the solid increases with pH even without cadmium, but this effect is increased in the presence of cadmium. The composition tends to the stoichiometric value of 20 eq. cations/mol apatite for the highest pH and [Cd](s) values. The sorption of cadium is therefore controlled by two processes : cationic exchange and protonation-deprotonation.