Cation exchange associated with displacement of aqueous solutions is a complex physicochemical process. In this study column experiments were combined with numerical modeling to determine the controlling chemical processes for natural systems at near-neutral pH. The experimental results showed the importance of two processes in addition to exchange between free cations: exchange of monovalent metal-bicarbonate complexes and proton buffering. The former primarily results from changes in CO2 pressure, whereas the latter results from changes in aqueous pH. Neglecting these two processes had little effect when modeling the cation elution curves. However, the pH and alkalinity breakthrough curves were poorly fit, since nonreactive behavior of these variables was assumed. Inclusion of either specific sorption of Ca and Mg hydroxy complexes or transfer of Al hydroxy species to or from the exchanger sites resulted in a better fit. It is shown theoretically that the two additional processes have a major impact on calcite equilibria when considering calcareous systems. ¿ American Geophysical Union 1993