SUMMARYCounter ionic migration rates of dilute Cs+and Sr2+against sodium in a Na‐montmorillonite gel are measured using a radially perfused diffusion cell. Enhanced or surface diffusion for caesium is observed both at ambient and elevated temperature. To quantify these surface diffusivities the ion‐exchange properties of the diffusing species are required. Thus, exchange isotherms for caesium against sodium on montmorillonite at 22 and 90°C are given for 10−1, 10−2, and 10−3M background NaCl concentrations. Caesium counterion surface diffusivities at 22 and 90°C are 2.2*10−6and 8.0 * 10−6cm2s−1, respectively. These values are found to be essentially independent of background ionic strength over the range 10−3to 10−1mNaCl. Experimental evidence of enhanced transport through Na‐montmorillonite for the divalent cation Sr2+is also confirmed with a surface diffusivity of 2 * 10−6cm2s