The amphoteric properties of hydrous fluorapatite surfaces have been investigated. The measurements were performed as potentiometric titrations in 0.1M NaCl medium at 25°C using a glass electrode. The experimental data could be explained by a model that assumes the surface equilibria equivalent P-O- + H+ = equivalent P-OH log β101s(int.) = 6.6 ± 0.1 (3σ) equivalent Ca-OH2+ - H+ = equivalent Ca-OH log β-110s(int.) = -9.7 ± 0.1 (3σ). The equilibrium constants were evaluated using the computer program FITEQL by Westall (Report 82-01, Department of Chemistry. Oregon State University, Corvallis, 1982), assuming a constant capacitance model for the electric double layer. The acidity constants result in a value of log [H+]pzc = -8.15, which shifts to log [H+]pzc = -7.13 upon exposure to atmospheric carbon dioxide