Spinel LiNi0.5Mn1.5O4 (LNMO) is a promising cathode material for high energy density lithium-ion batteries (LIBs), but further enhancement of LNMO electrochemical performance requires a better understanding of its intrinsic surface properties. Herein, we employ first-principles calculations to obtain insights into the transition-metal disproportionation reactions and oxidation states at the LNMO (001) and (111) surfaces examining the role of crystal surface, state of charge (lithium content), oxygen vacancies, and protonation of surface oxygen atoms. Our results reveal possible coexistence of multiple transition-metal oxidation states on the (001) facet promoted by surface protonation and presence of oxygen vacancies. This should facilitate Mn dissolution, while it is not the case for the (111) surface. (C) 2018 The Electrochemical Society.