The complexation of Alizarin Red S (ARS) at the surface of hydrous fluorite particles has been investigated by means of potentiometric titrations, adsorption experiments, and xi-potential measurements in 0.1 mol dm(-3) KCl ionic medium at 25.0 degrees C, as well as by UV/visible specular reflectance, FT-IR, and FT-Raman spectroscopy. Chemical reaction models describing the equilibria of ARS (HA(2-)) at the aqueous fluorite surfaces (=X) have been established as follows: =X + HA(2-) + H+ double left right arrow =XH(2)A(-), log beta(111)(s)(int) = 16.3 +/- 0.1 (3 sigma) =X + HA(2-) double left right arrow =XHA(2-), log beta(011)(s)(int) = 9.8 +/- 0.1 (3 sigma) =X + HA(2-) double left right arrow =XA(3-) + H+, log beta(-111)(s)(int) = 2.8 +/- 0.1 (3 sigma). Experimental data were evaluated using the computer program FITEQL on the basis of a constant capacitance model for the electric double layer. Surface complexation mechanisms involving the R-SO3-, R-Beta-OH, and R-alpha-OH active groups of the ARS molecule are proposed to describe coordination to the fluorite surface.