The diffusion coefficients of the constituent ions were measured in CsBr and CsI single crystals between 300°C and 550°C by means of radioactive tracers and were compared with electrical conductivity measurements. Approximate satisfaction of the Nernst-Einstein equation indicates that the conductivity is nearly completely ionic and the diffusion measurements show that the halogen ion defects are the more mobile. Attempts to identify the mechanisms for ionic transport by means of the Bardeen-Herring correlation factor are discussed. Schottky defects seem likely but the assumption of an additional mechanism for cationic transport is required. If Schottky defects are predominant their formation enthalpies are 2.0 ev and 1.9 ev in CsBr and CsI respectively, and the activation enthalpies for halogen vacancy motion are 0.27 ev and 0.3 ev respectively, while the cesium vacancy activation enthalpy is about 0.58 ev for both salts.