Carbonate-based electrolytes used in Lithium-ion electrochemical cells were found to undergo ester exchange reactions, where the use of dimethyl carbonate and diethyl carbonate resulted in the in situ formation of ethyl methyl carbonate. The reaction was found to be reversible and occurs during the first charge cycle of the LiCoO2/petroleum coke lithium-ion system. Mechanistic studies were carried out and determined that the ester exchange reaction is reductively initiated at the carbon anode. A mechanism has been deduced, with an intermediate alkoxide species responsible for the ester exchange reaction. Electrode passivation was found to limit the extent of the reaction during subsequent cycles, with the choice of electrolyte solvent impacting the passivation of the electrode.