Manganese-rich olivine LiFe0.33Mn0.67PO4 was proven to be an interesting electrode material for Li-ion batteries. In this paper we focus on its electrode/electrolyte reactivity that could play a role in aging processes of LiFe0.33Mn0.67PO4 // graphite cells. Positive and negative electrodes of LiFe0.33Mn0.67PO4 // graphite cells were analyzed by X-Ray Photoelectron Spectroscopy (XPS) at different steps of the first electrochemical cycle to investigate the influence of the working potential increase of this material (as compared to LiFePO4) on the formation of passivation films at the surface of both electrodes. The higher working potential of LiFe0.33Mn0.67PO4 causes an increased reactivity toward the electrolyte with a redox process between Mn3+ and carbonates solvents at the outermost surface of the electrode material. It results in a thicker interface layer with respect to LiFePO4. However this interface remains very thin. Some influence on the passivation film formed at the graphite surface was also evidenced. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.019303jes] All rights reserved.