Abstract The diffusion of Na22Cl in RbCl was measured in the temperature range 377–707°C by the tracer-sectioning technique. The activation energy is 2.06± 0.02 and 0.59 ± 0.01 eV in the intrinsic and extrinsic regions, respectively. The temperature dependence of the correlation factor, as deduced from isotope-effect measurements, is 0.19 eV in the intrinsic region and -0.05 eV in the extrinsic region. When a theoretical value for the Na+- vacancy binding energy is used, values of ½hf =1.22, eV and hm= 0.55 eV are obtained where hf is the energy of formation, and hm the energy of motion, of a cation vacancy. These values are not in agreement with the calculation by Tosi and Doyama.