High-temperature storage of the lithium manganese spinet cathode was examined for Li1.02-xMn1.98O4 using 1 M LiPF6 in EC:DEC (1:1) electrolyte solution. The structure, magnetic properties, and valence state of Mn were determined before and after the storage using Mn K-edge XANES, X-ray diffraction, SQUID, ICP spectroscopy, and electrochemical measurements. After the storage at 80degreesC for 6 days, single-phase property was observed at x=0 and 0.96 in Li1.02-xMn1.98O4, while multi-phase property was observed between the compositions, x=0.18 and 0.63. Shallow de-lithiated region near x=0.18 was easily affected by the storage; the Li/Li1.02-xMn1.9O4 cell showed large capacity failure from 111 to 40 mAh/g after the storage, which corresponded to the phase transition with the increase in the valence state of Mn after the storage. Magnetic measurement was found to be quite sensitive and effective to detect the subtle structural changes caused by the high-temperature storage. (C) 2003 Elsevier Science B.V. All rights reserved.