Pure single crystals have been used for measurements of ionic conductivity (160° -425 °C ) , CI diffusion (275° - 415 °C ) , and Tl diffusion (290° - 400 °C ) . Conductivity measurements were made with a capacitance bridge at 1 to 10 kHz, and diffusion coefficients were obtained with radioactive tracers and the sectioning technique. It is found that D*(Cl) is 10 to 40 times larger than D* (Tl) . An empirical fit gives D*(CI)=0.031 exp [ - (0.771 ± 0.010) /k T] , D*(Tl)=0.62 e x p [- (1.101 ± 0 . 0 2 0 ) /k T] with D in cm2/s and the energy in eV. The conductivity has an activation energy of 0.76 eV for 230°-330 ° C , increasing to 0.87 eV for 375° - 425 ° C . For several reasons the conductivity is believed to be entirely ionic. The Nernst-Einstein relation is generally well obeyed, but there is a significant excess diffusion of 10 t o 15%. The results show that the defect structure consists primarily of Schottky defects with highly mobile anion vacancies. The excess diffusion can be accounted for by the presence of up to 20% of either vacancy pairs or next nearest neighbor jumps into single vacancies; there is no clear preference for either mode. With a Schottky formation energy of 1.36 + 0.03 eV from other work, the mobility energies are 0.104 ± 0.005 eV for CI for either model, and 0.56 ± 0.04 or 0.44 ± 0.02 eV for Tl, respectively. Comparisons are made to observations of conductivity and diffusion by other workers near r o om temperature and in the melt.