The acid-base properties of hydrous fluorite surfaces and ion exchange reactions between surface lattice ions of F- and OH- ions in bulk solution have been investigated. The measurements were performed as potentiometric titrations with varied solid concentrations in 0.1 mol dm(-3) KCI medium at 25.0 degrees C using a glass electrode for -log[H+] and a fluoride ion selective electrode for -log[F-]. The experimental data could be explained by a model that assumes the surface equilibria [GRAPHICS] log beta(-210-1)(S)(int)= -20.3 +/- 0.1 The equilibrium constants were evaluated using the computer program FITEQL assuming a constant capacitance model for the electric double layer.