Ba2In2O5 is one of the oxygen deficient ceramic materials widely investigated for both oxygen ion conduction and proton conduction in oxidizing atmospheres. However, its electrochemical properties have not been studied in hydrogen containing atmospheres. In this work, the electrical conductivity of Ba2In2O5 in hydrogen containing atmospheres was investigated by ac impedance spectroscopy in the temperature range between 100 and 500 degrees C and compared to its conductivity in air and nitrogen in order to estimate the contribution of proton conductivity to the total conductivity. A stable electrical conductivity of over 0.3 S/cm was achieved in the temperature range of 300-480 degrees C in a 50% vol H-2/50% vol N-2 atmosphere. Electromotive force measurements (and complimentary open circuit cell voltage measurements) revealed high proton transport numbers over the temperature range. Ba2In2O5 shows chemical stability in hydrogen containing atmospheres at temperatures up to 480 degrees C, while decomposing at higher temperatures. Mechanical instability was noticed in humid atmospheres at 400 degrees C and above. This previously unreported high conductivity in H-2 atmosphere creates an opportunity for use of Ba2In2O5 as a proton conductive material for a range of intermediate temperature electrochemical devices. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3511787] All rights reserved.