Comprehensive experiments are carried out in order to gain insight into the effect of manganese contamination of the solid electrolyte interphase at the anode in Li-ion batteries. Electrochemistry shows that surface films contaminated with Mn are not passive toward electrolyte decomposition and are electroactive. Soft X-ray spectroscopy shows that the Mn is at +2 oxidation state in the film, just like Mn ions in the electrolyte. We believe Mn2+ from the electrolyte reduces to Mn-0 at the electrode surface and Mn-0 further reoxidizes by reacting with solvent molecules. Although not detected in soft X-ray spectra, it is possible that Mn-0 is present in an amount high enough to allow for an electron leakage through the film, thereby suppressing passivation. Mn trapped in the film reversibly reacts with lithium according to a conversion reaction. It is a multiphase transformation yielding severe structural changes, hence the film undergoes severe morphological changes during cycling, thereby favoring electrolyte decomposition and further film growth. In this regard, Mn contamination of the SEI therefore promotes a larger cyclable lithium loss at the anode as compared to a Mn-free SEI, and thereby a persistent capacity loss of graphite/LMO cells. (C) 2013 The Electrochemical Society. All rights reserved.