The coexistence of LiMn 2 O 4 and Li 2 MnO 3 phases from MnO/Li 2 CO 3 mixtures with lithium cationic fraction 0.33≤x≤0.53 was studied by X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) measurements. XRD quantitative phase analysis was carried out using the Rietveld profile refinement procedure, while the Li 2 MnO 3 percentage was determined directly by comparing the EPR intensities with the intensity of the pure Li 2 MnO 3 spectrum. The experimental results show the presence of an Li-rich spinel phase and allow an estimation of the lithium excess, since the structural analysis of pure Li 2 MnO 3 samples evidenced a nearly stoichiometric composition (x=0.663) with very limited cationic disorder.