Several compositions of the cubic spinel Li1+xMn2-xO4-delta phase in the lithium-manganese-oxygen (Li-Mn-O) system were synthesized at 700, 750, and 800 degrees C in air (p(O2) = 0.2 atm) to investigate the Li-rich boundary of the cubic spinel phase at these temperatures. The lattice parameters of the several compositions were determined by Rietveld analysis of the measured X-ray patterns, and the Li and Mn contents of the samples were measured using inductively coupled plasma with optical emission spectroscopy (ICP-OES). A Vegard-like dependence of the measured lattice parameter of the cubic spinel phase with Li to Mn ratio exists in the homogeneity range of the cubic spinel. This dependence could be used to derive the boundary of the single phase cubic spinel field in the Li-Mn-O system at 700 and 750 degrees C at p(O2) = 0.2 atm and to estimate the Li-rich boundary at 800 degrees C. The results of the present study are compared with two other experimental studies on the homogeneity range of the cubic spinel phase in an attempt to resolve the contradiction between these two studies.