The enthalpies of formation of Li1+x Mn2−x O4 (0< x <0.1) spinel phases were measured using high temperature oxide solution calorimetry in a sodium molybdate solvent at 701°C. One commercially available LiMn2O4 powder and two Li1+x Mn2−x O4 samples synthesized using the sol–gel method were used for the measurements. The enthalpies of formation of the spinels from the binary constituent oxides and from the elements become more exothermic with increasing lithium content and increasing average oxidation state of the manganese cation in the range 0< x <0.1. Additionally, the lithium-rich boundary of the Li1+x Mn2−x O4 phase for 0< x <0.1 was re-investigated using differential thermal analysis combined with thermogravimetric analysis.