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The diffusion coefficient of the chlorine ion was measured in pure single crystals of potassium chloride and in crystals containing mole fractions of strontium chloride between 2.1 x 10-5 and 4.69 x 10-4. The results were consistent with the assumption that both anion vacancies and vacancy pairs contribute to the diffusion of the chlorine ion. The diffusion coefficient of the chlorine ion in pure potassium chloride was well represented by the equation D=61 exp (-2.12±0.05 eV/kT) cm2/sec from 560 to 760 °C. The anion-vacancy contribution to the chlorine-ion diffusion coefficient in pure potassium chloride was Dα=36.5 exp[-(2.10 eV)/kT] cm 2/sec and the mole fraction of Schottky defects was n0=43.9 x exp[-(2.31 eV)/2kT]. The jump activation energy of the anion vacancy was 0.95 eV. The vacancy-pair contribution to the diffusion coefficient of the chlorine was D ρ = 8.56 x 103 exp[-(2.65eV)/kT] cm2/sec. These results are in good agreement with theory. Below 560 °C, experimental diffusion coefficients in pure potassium chloride were too large to be interpreted in terms of anion-vacancy and vacancy-pair diffusion. Similar, anomalous behavior was found below 645° C in crystals containing 469 ppm of strontium.