Structural investigation was performed on a range of NMC materials, Li1+x(Ni1/3Mn1/3Co1/3)(1-x)O-2 (0 <= x <= 0.13), using in situ X-ray synchrotron diffraction while electrochemically cycling them between 2.5 and 5.0 V vs. Li/Li+. From the analysis of the obtained data, an irreversible phase transformation of part of the starting O3 phase (R (3) over bar m) to the O1 phase (P (3) over bar m1) was observed for all the NMC samples, where the O1 phase was detected at more negative potentials with increasing overlithiation content. Moreover a second irreversible reaction was detected at positive potentials using ex situ X-ray synchrotron diffraction for overlithiated NMC (x = 0.1) that involved loss of Li2MnO3 ordering in the structure. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3607982] All rights reserved.