A synthetic fluorapatite was prepared that undergoes a phase transformation generated during a dialysis step. A surface layer with the composition Ca-9(HPO4)(2)(PO4)(4)F-2 is formed, which is suggested to form as one calcium atom is replaced by two protons. A surface complexation model, based upon XPS measurements, potentiometric titration data, batch experiments, and zeta-potential measurements was presented, The CaOH and OPO3H2 Sites were assumed to have similar protolytic properties as in a corresponding nonstoichiometric HAP (Ca-8.4(HPO4)(1.6)(PO4)(4.4)(OH)(0.4)) system. Besides a determination of the solubility product of Ca-9(HPO4)(2)(PO4)(4)F-2, two additional surface complexation reactions were introduced; one that accounts for a F/OH ion exchange reaction, resulting in the release of quite high fluoride concentrations (similar to 1 mM) that turned out to be dependent on the surface area of the particles. Furthermore, to explain the lowering of pH(iep) from around 8 in nonstoichiometric HAP suspensions to about 5.7 in FAP suspensions, a reaction that lowers The surface charge due to the readsorption of fluoride ions to the positively charged Ca sites was introduced: CaOH2+ + F- -> CaF + H2O. The resulting model also agrees with predictions based upon XPS and ATR-FTIR observations claiming the formation of CaF2(S) in the most acidic pH range.