Eleven types of manganese-containing electrode materials were subjected to long-term storage at 55°C in 1M LiPF6 ethylene carbonate/dimethyl carbonate (EC/DMC) solutions. The amount of manganese dissolution observed depended upon the sample surface area, the average Mn oxidation state, the structure, and substitution levels of the manganese oxide. In some cases, structural changes such as solvate formation were exacerbated by the high temperature storage, and contributed to capacity fading upon cycling even in the absence of significant Mn dissolution. The most stable materials appear to be Ti-substituted tunnel structures and mixed metal layered oxides with Mn in the +4 oxidation state.