We investigate the reversible and irreversible transformations, the Li, Mn spinel undergoes under different atmospheres (air, O 2 , and N 2 ) when heated up to 1050 o C. In air and O 2 , a substantial reversible cation exchange occurs. For T>=800 o C, Mn 2 + ions substitute Li + at the tetrahedral (8a) position, and Li + shifts to interstitial octahedral (16c) site. Charge balance is achieved by a decrease in the Mn 3 + fraction, which is partially reduced to Mn 2 + in the regular octahedral (16d) site, according to the charge distribution (Li 1 - 2 y + Mn 2 y 2 + ) 8 a [Li 3 y + ] 1 6 c [Mn 5 y 2 + Mn 1 - 5 y 3 + Mn 1 4 +] 1 6 d O 4 (y increases with T). Under N 2 flow, a first decomposition occurs between 600 and 800 o C and yields Mn 3 O 4 , orthorhombic LiMnO 2 and O 2 . A minor O 2 release occurs when heating above 900 o C, which is accompanied by the transformation of o-LiMnO 2 into a cubic Li x Mn 1 - x O solid solution (x=<0.5), consistent with a decrease in the average oxidation state of Mn ions. This cubic phase is stable at high temperature and decomposes upon cooling (T=<800 o C) leaving just the Mn 3 O 4 and o-LiMnO 2 phases.