The diffusion coefficients for carrier-free ⁵⁴Mn in pure NaCl and KCl single crystals and ⁸⁵Sr in pure KCl single crystals were measured in the intrinsic conductivity region as functions of temperature. Results for Sr and Mn in KCl were unexceptional compared with results for other ions in the alkali halides but results for Mn in NaCl were characterized by unexpectedly large diffusion coefficients, a low activation energy (0.62 eV) region above 830K, and a region of larger activation energy below this temperature. These results were difficult to rationalize by the usual vacancy-diffusion model.