&NA;The rate at which water percolates in soils may have a considerable effect on the kinetics of transport processes. Undisturbed samples of a Luvisol subsoil that differed in their hydraulic conductivities (k1: 4 to 6 cm d−1; k2: 20 to 21 cm d−1; k3: 42 to 59 cm d−1) were leached under saturated conditions with 4, 20, 102, and 205 mMBaCl2at a hydraulic gradient of 6.5, and the Darcy velocities were q1 (270 to 360 mm d−1), q2 (1280 to 1360 mm d−1) and q3 (2730 to 3830 mm d−1). Objectives were to investigate how different flux rates affect the amount of desorbed cations in an undisturbed soil when percolated with BaCl2solutions of different concentrations and whether ion concentrations in the percolates can be predicted. The model PHREEQC was used for the calculation of one‐dimensional transport, inorganic complexation, and multiple cation exchange, and the program UCODE was used for the parameter optimization. The best prediction performance of ion concentration and the sum of cations desorbed at q1 (73% satisfactory or good predictions) was obtained when the parameters were 0.6 cm (dispersivity &lgr;), 0.74 (fraction of mobile water f&thgr;), 3.4e‐5 s−1(mass transfer coefficient &agr;), and 0.91 (proportion of cation exchange capacity in contact with the mobile water fCEC‐m). At q2, the optimum performance (71% satisfactory or good predictions) was achieved with &agr; = 6.2e‐4 s−1and fCEC‐m= 0.58. At q3, the optimum parameters were &agr; = 6.8e‐4 s−1and fCEC‐m= 0.38, which resulted in 58% satisfactory or good predictions. This study suggests that &agr; increases with increasing pore water velocity and that fCEC‐mdecreases with increasing hydraulic conductivity. Application of UCODE and PHREEQC contributed to improved understanding of cation exchange reactions in undisturbed soil. (Soil Science 2003;168:253‐266)