Thallium bromide (TlBr) is attractive for high energy radiation detection, given its large molecular weight and wide energy bandgap. However, TlBr exhibits levels of ionic conductivity that can lead to an undesirable leakage, or dark current, thereby reducing sensor performance. To investigate the role of dopants in controlling the ionic conductivity, single crystals of TlBr were grown using zone refining and/or vertical Bridgman methods with controlled levels of donor (Pb) dopants. Their electrical properties were examined as a function of temperature (20-300 degrees C) with frequency dependent impedance spectroscopy. A Schottky-based defect equilibria model was fitted to the resulting conductivity data, and enthalpies of Schottky defect formation (0.91 +/- 0.03 eV), cation migration (0.51 +/- 0.03 eV), and anion migration (0.28 +/- 0.05 eV) were extracted. Br vacancies were found to posses about 5 orders of magnitude higher mobility than that of Tl vacancies at 20 degrees C. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3525243] All rights reserved.