The atomic mass ratio of ca. 2 between deuterium and hydrogen is the highest for any pair of stable isotopes and results in significant and measurable HID isotope effects in high temperature proton conductors containing these species. This paper discusses HID isotope effects manifested in O-H/O-D vibration frequencies, the mobility of H+/D+ carriers, the kinetics of the electrochemical oxidation of H-2/D-2, the solubilities of H2O/D2O and, finally, the spontaneous electromotive force that appears across H-2/D-2 cells with proton conducting electrolytes. Comparable work on tritium-exchanged materials is also discussed. The results highlight the usefulness of isotope effects in the study of high temperature proton conductors. (C) 2015 Elsevier B.V. All rights reserved.