Two kinds of spinel structure lithium manganese oxides obtained by a melt-impregnation method were examined in a lithium nonaqueous cell. The first type shows a voltage profile of a typical spinel electrode with the characteristic two-step process, which delivers a larger capacity, but fading capacity on cycling (type I). The other type delivers a slightly lower capacity and has ideal rechargeability with a quasi-one-step process (type II). The mechanism of the lithium ion insertion into the spinel compound was studied by several physical and chemical methods (differential chronopotentiometric curves, cyclic voltammetry, and open-circuit voltage measurement). X-ray diffraction was also used to investigate the structural changes for both types of compounds at different oxidation depths at first charge. The results show that a two-phase reaction (cubic a(0) = 8.154 Angstrom and a(0) = 8.072 Angstrom) occurred in the range of 0.1 < x < 0.45, and a one-phase reaction [cubic a(0) = 8.163 to 8.247 Angstrom] occurred in the range of 0.45 < x < 1.0 for the first charge for the type I spinel. A one-phase reaction (cubic a(0) = 8.105 to 8.223 Angstrom) was observed over the entire intercalated range of 0.25 < x < 1.04 for the type II spinel compound. The capacity fading on cycling for the type I spinel occurs only in the range of 0.1 < x < 0.45, where the shape of the charge and discharge curves in this range was gradually changed to an S-shape from and L-shape during cycling. This is attributed to the fact that the two-phase structure is transformed to more stable one-phase structure.