The stability field of the Li-Mn-O spinel, Li1 + x Mn2 -x O4 -delta, was investigated as a function of temperature, T, cation composition, n (Li)/n (Mn), and oxygen partial pressure, pO(2), by means of in situ X-ray diffraction (XRD) and thermogravimetry (TG). In a T-n (Li)/n (Mn) phase diagram, the stability field is described by the upper and lower critical temperatures, T (c1) and T (cL), respectively. Above T-c1, Li2MnO3 is formed as a second phase, and below T (cL) Mn2O3 is formed. Both T (c1) and T (cL) decrease continuously with increasing n (Li)/n (Mn) and increase with increasing pO(2). The single phase region contains lithium-deficient and lithium-excess spinels, and no discontinuous change of the critical temperature curves was found at n (Li)/n (Mn) = 0.5, corresponding to LiMn2O4. With the experimental data obtained in this work, a three-dimensional stability field of the Li-Mn-O spinel phase diagram is put forward to describe the relationship between T, n (Li)/n (Mn) and pO(2). In addition, the upper critical temperature, T (c1), was investigated for spinels doped with Ni, Co and Mg. For all three dopants, T (c1) decreases with increasing dopant concentration.