The diffusion of Kr in RbCl single crystals was studied following Kr implantation at 15 keV and doses of 3.7 × 1013to 3.7 × 1015ions/cm2. The compound exponential depth distribution was utilized by comparing ordinarily implanted specimens with others having the outermost ∼ 2000 Å removed inn-butanol so as to leave only the deeply channeled portion of the incident beam in an ambient of low damage concentration. Two diffusion mechanisms were found, one at 150–300 °C, with an activation enthalpy of 1.21 ±0.07 eV, and one at 300–450 °C, with an activation enthalpy of 2.3±0.2 eV. The first process corresponds reasonably well to Norgett and Lidiard's description of interstitial diffusion in the presence of extrinsic or collisionally-produced single-vacancy traps, for which an activation enthalpy of 1.28–1.78 eV was calculated. The second process is not so easily interpreted but possibly corresponds to interstitial diffusion in the presence of small vacancy clusters or small gas clusters, with or without concurrent radiation-damage annealing. A comparison of the pre-exponential factors for the first process as a function of dose allows the single-vacancy concentrations in the bombarded regions to be estimated, and evidence is presented that the concentration saturates at about 2 × 10−2±1atomic fraction for doses in excess of 5 × 1015ions/cm2. It is shown that the interstitial frequency factorr0works out to about 1 × 1012±lsec−lon the basis of the proposed diffusion mechanism. This is a reasonable value and tends to confirm the model.