The influence of the anionic substitution and the thermal treatment on the acid/base properties of hydroxyapatite was studied by temperature-programmed desorption of ammonia (TPD-NH3). Stoichiometric hydroxyapatite, A-type and B-type carboapatites have been synthesized. Substitution of OH- by the CO32- (Carbo-A) modifies the acid strength distribution, increases the overall acidity and acid site density (mu mol/m(2)). Substitution of the PO43- (B-site) by the CO32- did not lead to noticeable changes on the adsorption sites. These results suggest that the Ca" ions are the most significant sites for ammonia adsorption. (C) 2006 Elsevier B.V. All rights reserved.