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 80degreesC for 24 h, 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. (C) 2003 Elsevier Science B.V. All rights reserved.