Dissolution of spinel manganese oxides and the concomitant cathodic capacity losses were examined in 4 V Li/PC + DME + LiClO4/LixMn2O4 cells where PC is propylene carbonate and DME is dimethoxyethane. Dissolved Mn2+ contents in the electrolytes were analyzed as a function of cathode potential and carbon contents in the composite cathodes. Characteristically manganese dissolution was notably high at the charged state (at >4.1 V vs. Li/Li+), in which potential range an electrochemical oxidation of the solvent molecules was also prominent. From this and another observation whereby the Mn dissolution increased with increasing carbon content in the composite cathodes, it was proposed that, at the charged state of the cathode the solvent molecules are electrochemically oxidized on carbon surfaces and an as-generated species promotes the manganese dissolution. Results of an ac impedance study revealed that Mn dissolution brings about an increase in contact resistances at the Mn-depleted spinel/carbon interface, and also in the electrode reaction resistances for Li+ intercalation/deinterealation. Thus, the Mn dissolution causes capacity losses in two different pathways; material loss of the loaded spinel and polarization loss due to a cell resistance increment. The former prevailed when cathodes contained excess amounts of carbon, while the latter became more of a problem as the carbon contents decreased.