Xenon‐133 diffusion coefficients were measured in single crystals of KI, RbI, and CsI over the temperature range of 150 to 500 °C. Xenon was incorporated in the crystals both by doping with precursor I133 and by fission recoil from an external fissionable source. Consistent diffusion results were obtained which yielded single values for the diffusion activation energy over the temperature range of measurement. A gas concentration effect which was attributed to gas atom clustering was observed at concentrations as low as 10−9 gas atom fraction. Diffusion results were found to be most consistent with a model in which gas atoms associate with mobile defect clusters and diffuse with the clusters through the crystal lattice.