Cation- and anion-exchange capacities (CEC and AEC) of Andisols vary with ionic concentration, C, and pH of the soil solution. Predicting retardation of ion transport in the soils using measured CEC and AEC is not straightforward under varying C conditions, for CEC and AEC are usually of different magnitudes from each other, and this could be erroneously taken as implying different retardations for cations and anions. The objective of this study was to predict retardation of Ca²⁺ and Cl^- transport in a homoionic Andisol under varying liquid-phase CaCl₂ concentrations. Miscible displacement experiments were conducted in a repacked humic Andisol column saturated with Ca²⁺ and Cl^-, to which stepwise changes in the influent CaCl₂ concentration were imposed. Electrical conductivity of the bulk soil at different depths as well as Ca²⁺ and Cl^- concentration in the effluent was determined. Although CEC of the soil was much larger than AEC, Ca²⁺ and Cl^- moved at the same velocity satisfying the electroneutrality condition, with both retarded relative to water. In addition, pH of the effluent was not constant but increased as the ionic concentrations decreased. These results show that pH changes associated with the changes in C must be taken into account in predicting retardation of ion transport from the measured CEC and AEC. Adopting pH-dependence implicit formulation of CEC and AEC (Katou, 2002), adsorption isotherms for Ca²⁺ and Cl^- were derived from the CEC and AEC of the soil measured by the repeated washing method as a function of C and pH. In this formulation, pH change is assumed to take place so that there are simultaneous and equivalent changes in CEC and AEC. The isotherms successfully predicted the retardations of Ca²⁺ and Cl^- transport observed in the displacement experiments.