The retention properties of a synthetic carbonate fluoroapatite and a natural francolite are compared in the present work from an investigation of the sorption of Ni-63 at tracer scale amounts onto these solids, Two different surface complexation models were successively used to fit the experimental adsorption isotherms obtained under various experimental conditions: the nonelectrostatic model and the constant capacitance model. The results are essentially described by two main equilibria involving one proton in acidic media and three protons in basic media. The corresponding thermodynamic constants are in agreement for both models. Modeling gives also close values for both solids, despite their distinct solubility and surface acidity.