The apparent diffusion coefficient, De, of a cation diffusing in soil was proposed to be Dem = [Dsm(dCs/dCsa} + 1Dam (dCa/dCsa)] / [1 + dCp/dCsa] where Dsm and 1Dam are the diffusion coefficients of the cation in the electrolyte-solution phase and in the adsorbed phase (exchangeable cations), respectively, and where the frame of reference in each case is with respect to the entire soil mass. The symbols Cs, Ca and Cp are concentrations of electrolyte-solution cations, adsorbed cations, and precipitated or immobilized cations, respectively; Csa is defined to be the sum of Cs and Ca. The concentrations are expressed with respect to the total volumetric water content of the system. The quantities, dCs/dCsa, dCa/dCsa, and dCp/dCsa were evaluated in experiments independent of the measurement of Dem. The relative contributions of Dsm and 1Dam to Dem were evaluated over a large range of Csa values for Cu2+ diffusing in water-saturated kaolinite and montmorillonite. In kaolinite clay with a cation exchange capacity (CEC) of 0.065 meq g-1, the percent of total diffusion due to diffusion in the adsorbed phase was 100, 52, 34, 22, 18, 9, and 7 at Csa concentrations of 0.015, 0.02, 0.03, 0.04, 0.05, 0.06, and 0.08 meq g-1, respectively. In montmorillonite clay with a CEC of 0.70 meq g-1, the percent of total diffusion due to diffusion in an adsorbed phase was 100, 57, 42, 34, 28, 20, and 13 at Csa concentrations of 0.03, 0.05, 0.10, 0.15, 0.20, 0.40, and 0.70 meq g-1, respectively.