The capacity loss and coulombic efficiency of commercial LiCoO2/graphite Li-ion cells have been examined using high precision coulometry and long-term cycling tests. The experiments show that time, not cycle count, was the dominant contributor to the degradation of LiCoO2/graphite Li-ion cells cycled at low rates and elevated temperatures. The differential voltage versus capacity, dV/dQ vs Q, of the cells was measured for all cycles during the extended cycling and fit using predicted dV/dQ vs Q plots calculated from Li/graphite and Li/LiCoO2 cells made from the same electrodes that were used in the commercial cells. From this analysis it was possible to determine fraction of positive and negative electrode masses that remained active as a function of cycle number and also the portion of capacity loss due to the relative slippage of the positive and negative electrode potential-capacity curves. A rapid impedance rise was observed near the end of the cycling testing. These results provide a model procedure for understanding of the failure of lithium-ion cells subjected to sustained high temperature cycling. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.056206jes] All rights reserved.