The solid/water interface reaction is investigated for the sorption of the Eu3+ ion onto a well-characterized natural hematite at pH </=6 in 0.1 mol/L NaClO4. Sorption isotherms are determined at different pH. The sorption of Eu3+ onto hematite at coverages <1% of the surface hydroxyl groups follows the ideal Nernst behavior and can be interpreted in terms of the surface complexation by the following equilibrium reaction: &tbond;XOH + Eu3+ if &tbond;XOEu2+ + H+ (log Kintx = 2.50 +/- 0.19). Complexation constants are calculated by either graphically interpretating the sorption isotherms or fitting the data using the code FITEQL. At higher surface coverages a deviation from the Nernst behavior is observed which can be described satisfactorily by a two-site surface complexation model, taking into account the lateral electrostatic interaction of surface-bound Eu(III): &tbond;YOH + Eu3+ if &tbond;YOEu2+ + H+ (log Kinty = -0.82 +/- 0.11). The experimental results are compared with data published in the literature, and the interpretation of sorption isotherms with regard to the sorption mechanisms is critically discussed. Copyright 1998 Academic Press.