The surface acid/base properties of magnetite (Fe3O4) particles and the sorption of Mg2+ onto magnetite and maghemite (gamma-Fe2O3) have been studied using high precision potentiometric titrations, batch experiments, and zeta potential measurements. The acid/base properties of magnetite were found to be very similar to maghemite except for the difference in surface site density, N-s(sites nm(-2)), 1.50 +/- 0.08 for magnetite, and 0.99 +/- 0.05 for maghemite. The experimental proton exchange of the magnetite surface increased from pH 10 and above, indicating dissolution/transformation reactions of magnetite at alkaline conditions. Thus, magnetite with its Fe(II) content proved to be less stable toward dissolution in comparison with pure Fe(III) oxides also at high pH values. Three different ratios between surface sites and added Mg2+ were used in the sorption experiments viz. 0.5, 1, and 2 Mg2+ site(-1). Surface complexation modeling of the Mg2+ sorption onto maghemite and magnetite was restricted to pH conditions where the interference from Mg(OH)(2)(s) precipitation could be ruled out. The model calculations showed that Mg2+ sorb onto the magnetite and maghemite surfaces as a mixture of mono- or bidentate surface complexes at 0.5 Mg2+ site and as monodentate complexes at 1 and 2 Mg2+ site(-1) conditions. Mg2+ was also found to adsorb more readily at the maghemite surfaces in comparison with magnetite surfaces. For experiments with excess Mg2+ relative to the number of surface sites, the calculations suggested the formation of polynuclear surface complexes on maghemite. (C) 2012 Elsevier Inc. All rights reserved.