Lithium manganese oxide is well known as a cheap and environmentally friendly electrolyte material in secondary lithium-ion-batteries. The spinel system used, (Li1+xMn2-yO4-d), ideally crystallizes in a cubic Fd-3m space group at room temperature. The cycling behaviour is of great industrial importance and it has been demonstrated that additional lithium, which is assumed to be incorporated into the spinel structure, increases cycling life drastically. In this work we focussed on the preparation of lithium manganese oxide spinel of high purity and different lithium excess contents. These samples were characterized by powder x-ray and powder neutron diffraction. X-ray absorption spectroscopy was applied to study the lithium incorporation in detail. The results have shown that excess lithium is built into the spinel structure and is most likely to be located on crystallographic sites noted 8b according to Wykoff-symbols in this space group. Additional temperature and oxygen partial pressure dependent investigations have been performed to elucidate the stability range of this spinel system.