First principles calculations are used to study the initial decomposition reactions of ethylene carbonate (EC) on the (111) surface of LiMn2O4 (LMO), a candidate for Li ion battery (LIB) cathode. Theoretical studies of interfacial reactions are particularly timely due to recent experiments on the effect of LMO crystal morphology on battery cyclability and interfacial film stability [J.-S. Kim, K. Kim, W. Cho, W. H. Shin, R. Kanno, and J. W. Choi, Nano Lett., 12, 6358-6365 (2012)]. We find that EC degradation is a two-step reaction. The first step is the rate determining reaction where a proton is abstracted from EC and transferred to the surface. The second step involves ring opening of the proton abstracted EC which turns out to have a smaller barrier. Both of these reactions are sensitive to the Li content, i.e. state of charge, of the model electrode. EC degradation via H-abstraction becomes a feasible reaction route when the cathode becomes more highly charged. Comparison with predictions on the (100) surface of LMO is discussed, and speculation on the growth of nanometer-thick interfacial layer on LMO despite its relatively modest operating potential are made in light of the reaction driving forces predicted in this work. (C) 2014 The Electrochemical Society. All rights reserved.