The electrochemical lithium-ion extraction/insertion properties of two spinel lithium manganese oxides, stoichiometric Li0.99Mn2O4 and lithium-rich Li1.04Mn1.93O4, were studied. Their voltage profile and cyclic voltammetry all show two-step reaction for lithium-ion extraction/insertion at their 4 V plateau. The high-resolution in situ X-ray diffraction (XRD) studies show both materials experience three cubic phases, cubic I, II, and III, during cycling at 4 V plateaus. Two two-phase coexistence regions were observed. These results show that phase transitions between cubic I and II and between cubic II and III are first-order transitions. The in situ XRD data also reveal that lithium-rich composition does not suppress the phase transitions of spinel between their three cubic phases, but leads to the increase of variation range of the lattice parameters of the cubic phases. This in turn reduces the jumping step of the lattice parameter between adjacent phases and increases the overlap of the adjacent phase. This study shows conclusively that the three-phase process is the intrinsic feature of spinel Li1+yMn2O4 during lithium-ion extraction/insertion on the 4 V plateau.