The empirical Rothmund-Kornfeld equation was used to describe a collection of cation exchange isotherms for K and Ca in Brucedale soil over a range of total solution concentrations (CT). A comparison of variable binary exchange selectivity coefficients (KKCa) which vary with CT and solution phase ion concentrations (CK, CCa) with constant KKCa values was thus possible. Percent differences between sorbed equilibrium concentrations calculated using variable and constant KKCa over a range of CT and CK were arranged into a contour map. Zones of potentially observable difference were limited, implying that simulations of cation transport using variable and constant KKCa values would be essentially identical. The predicted effect was examined using a numerical model for cation transport. The model successfully described observed spatial distributions for cation concentrations following constant-flux infiltration of electrolyte solution into soil columns. Simulations using variable KKCa values provided only a slight improvement over simulations using constant KKCa values, thus validating the use of the contour map as a convenient method to evaluate the need to use variable selectivities in simulations of cation transport. ¿ American Geophysical Union 1993