Diffusion coefficients of the chloride ion in NaCl have been measured between 300° and 700°C using an isotopic exchange method. The concentrations of cation impurities were deduced from electrical conductivity measurements over the same temperature range. In the extrinsic region, the diffusion coefficients do not decrease with increasing amounts of cation impurity to the extent required for diffusion by single vacancies.The general conclusion that diffusion occurs both by single vacancies and by vacancy pairs agrees with that reached earlier by Laurance. Analyses of the experimental results in the intrinsic region yield for single vacancy anion diffusionDa=1.1×e−1.92/kTcm2sec−1and for vacancy pair diffusionDp=363×e−2.37/kTcm2sec−1, and the activation energies, which are given in electron volts, agree well with theoretical estimates.Large variations were observed in the anion diffusion coefficients for crystals doped with Ca++; the cause cannot be established definitely.