We have studied capacity fadings of acid-treated lithium manganese oxides in high-temperature storage. The amounts of dissolved lithium and manganese ions and consumed acids were determined in the reaction of lithium manganese oxides with acids. Discharge capacities of the acid-treated samples were measured and the small capacity fadings were observed. X-ray diffraction patterns of the acid-treated samples were almost the same as the one before the acid-treatment. After the acid-treated samples were stored in the EC/DMC solutions without Mn and Li ions at 80°C for 24h, they gave no further capacity fadings. On the other hand, after the storage under the same condition but in the EC/DMC solution containing both Mn and Li ions, the same acid-treated samples as above showed large capacity fadings. Thus, the following mechanism is deduced; the acid-treated lithium manganese oxide would react with Mn and Li ions in the solution, and its spinel structure would partly be fractured to result in the severe capacity fading.