Batteries comprised of LiMn2O4 cathodes (LMO) and Li4Ti5O12 anodes (LTO) have potential advantages in terms of cost, safety, and power for transportation applications. In this manuscript, we present results from a cycle and calendar life study of 2 Ah LTO/LMO cells held at temperatures of 30 degrees C, 45 degrees C, and 60 degrees C for 5 months. The cells held at elevated temperatures had measurable loss of capacity and severe loss of power. The LTO anodes harvested from the cells were found to have developed a coating during testing comprised of manganese, phosphorous, and fluorine, which likely resulted in the decreased cycling and power performances. In addition, there was significant gas generation inside of the cells predominantly consisting of hydrogen. We report on the initial diagnostics of the power loss and gas generation for these LTO/LMO cells. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.013208jes] All rights reserved.