The X-ray diffraction analysis of Li1+delta Mn2-delta O4 having delta=0.00, 0.03, 0.06, 0.12, 0.18, 0.24, 0.33, 0.42, and 0.50 powders, synthesized by a sol-gel process, shows that a single-phase cubic spinel structure is stable only when delta < 0.24. When lithium content increases to delta>0.24, the percentage of monoclinic Li2MnO3 increases until delta=0.42 and saturates thereafter. On magnesium doping, the range of stability of cubic spinel Li1+delta Mn2-delta-0.1Mg0.1O4 decreases to delta < 0.18, and monoclinic Li2MnO3 starts to appear when delta>0.18. In magnesium-doped Li1+delta Mn1.9-delta-0.1Mg0.1O4, the ion Mg2+ goes to octahedral sites and decreases the population of Mn3+ in the 16d octahedral sites, and so the lattice parameter of the magnesium-doped cubic spinel for the same delta is lower than that for undoped Li1+delta Mn2-delta O4. However, at higher delta, the population of Mn3+ may get totally exhausted and the charge balance may be maintained by the creation of defects. The voltage step appearing in the discharge curve has been attributed to a transition from two cubic phases to one cubic phase. With increasing delta, the size of the step decreases and becomes broader. However, this voltage step does not appear to be affected by magnesium doping.