Lithium-rich spinel Li1.2Mn1.8O4 thin film electrodes operated at 3 V/Li+/Li are studied by means of XPS, mainly on the basis of the evolution of the Mn2p XPS peak during the electrode cycling. The anal. of this core peak has long been debated in literature given its complex character. Based on manganese oxide refs., MnO (Mn2+), Mn2O3(Mn3+) and Li2MnO3(Mn4+), we propose a deconvolution method to identify each Mn oxidn. state. This method is then used for the deconvolution of Mn2p XPS peaks of bulk lithium-rich spinels Li1+xMn2-xO4 (0 ≤ x ≤ 0.25) for validation before proceeding to the study of cycled Li1.2Mn1.8O4 thin film electrodes. Electrochem. measurements exhibit significant capacity loss during the first cycle. Based on XPS analyses, this phenomenon could be explained by mech. breakup of parts of the electrode. A stable behavior during subsequent cycles is then obsd. The presence of Mn2+ species (XPS) at the most top surface of the electrode and the significant polarization obsd. during the discharge illustrate the kinetical limitation of the two-phase reaction, despite the reduced thickness of the electrode material.