Acid-base reactions and surface complexation of Sr(II) at the hematite/water interface have been studied by means of potentiometric titrations at three different temperatures: 25, 50, and 75 degrees C. Equilibrium measurements were performed in 0.1 M NaCl. In the evaluation of equilibrium models for the acid-base reactions and complexation reactions in the three-component system H(+) &bond;(&tbond;FeOH)&bond;Sr(2+), the constant capacitance model was applied. During the titrations with Sr, aliquots of the suspension were sampled at in several points. The aqueous concentrations of Sr were analyzed by atomic absorption spectrometry. Treatment of data included tests for formation of both inner-sphere and outer-sphere complexes of different stoichiometric composition. The proposed equilibrium model consists of the following surface complexes of inner sphere type: &tbond;FeOHSr(2+) and &tbond;FeOSrOH. Besides the stability constants for the surface complexes, the thermodynamic parameters DeltaH and DeltaS were evaluated. The combined effect of a decrease in pH(pzc) with increasing temperature and positive enthalpies of surface complex formation favors adsorption of Sr at elevated temperatures. Copyright 1999 Academic Press.