The dissolution and surface complexation of a non-stoichiometric hydroxyapatite (Ca-8.4(HPO4)(1.6)(PO4)(4.4)(OH)(0.4)), (HAP) was studied in the pH range 3.5-10.5, at 25 degrees C in 0.1 M Na(Cl). The results from well-equilibrated batch experiments, potentiometric titrations, and zeta-potential measurements were combined with information provided by Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The information from the analyses was used to design an equilibration model that takes into account dissolution, surface potential, solution and surface complexation, as well as possible phase transformations. The results from the XPS measurements clearly show that the surface of the mineral has a different composition than the bulk and that the Ca/P ratio of the surface layer is 1.4 +/- 0.1. This ratio was also found in solution in the batches equilibrated at low pH where the dominating reaction is dissolution. In the batches equilibrated at near neutral pH values, however, the Ca/P ratio in solution attains values as high as 25, which is due to re-adsorption of phosphate ions to the HAP surface. The total concentration of protons as well as the total concentration of dissolved calcium and phosphate in solution were used to calculate a model for the dissolution and surface complexation of HAP. The constant capacitance model was applied in designing the following surface complexation model: CaOH + H+ reversible arrow CaOH2+ log beta(intr.) = 8.41 +/- 0.16 OPO3H2 reversible arrow OPO3H- + H+ log beta(intr.) = -1.11 +/- 0.13 CaOH + HPO42- + H+ reversible arrow CaOPO3H- + H2O log beta(intr.) = 11.63 +/- 0.15 OPO3H2 + Na+ reversible arrow OPO3Na- + 2H(+) log beta(intr.) = 11.08 +/- 0.12 In addition a solubility product with log beta = -23.27 +/- 0.29 was obtained: Ca-8.4(HPO4)(1.6) (PO4)(4.4)(OH)(0.4)(s) + 4.8H(+) reversible arrow 8.4Ca(2+) + 6HPO(4)(2-) + 0.4H(2)O Furthermore, this model predicts a pH(zpc) = 7.9, which is in agreement with pH(iep) = 8.1 obtained from zeta potential measurements. This model can be used as a helpful tool to predict the reactivity of HAP in different aquatic environments. (C) 2008 Elsevier Ltd. All rights reserved.