In this paper, we conducted potentiometric titrations, batch adsorption experiments and FT-IR analysis to study the uptake of copper in illite/water suspensions and then applied the constant capacitance surface complexation model to interpret the reaction mechanism at the aqueous illite surfaces. Our research shows that the copper adsorption at these surfaces is strongly dependent on pH and that the adsorption causes a deprotonation of surface groups. We propose that the uptake of copper in the carbonate-free illite suspensions can be explained by the formation of mononuclear surface complexes, =SOCu+ and =SOCuOH, and a multinuclear surface complex, =SOCu2(OH)(+)(2), followed by the formation of a bulk precipitate, Cu(OH)(2)(s), or a surface precipitate, =SOCu2(OH)(3)(sp). For the illite suspensions containing carbonates, we propose that the copper-illite interaction can be depicted by the formation of mononuclear surface complexes, =SOCu+ and =SOCuOH, followed by the formation of a copper hydroxylcarbonate precipitate, Cu-2(OH)(2)CO3(s), rather than a copper hydroxide precipitate. The existence of Cu-2(OH)(2)CO3(s) in the carbonate-containing illite suspensions was identified by FT-IR analysis.