Single phase Al or Mg-substituted Li1.05Mn1.95-zMzO4 (M=Al or Mg) (z=0, 0.05, 0.1, 0.15, 0.2) samples were synthesized by a two-step solid-state reaction. Electrochemical studies confirmed that Al or Mg substitution reduced the reversible specific capacity of Li1.05Mn1.95-zMzO4. The effects of Al or Mg substitution on the thermal stability of charged Li1.05Mn1.95-zMzO4 samples in the presence of a 1 M LiPF6 ethylene carbonate:diethyl carbonate electrolyte were studied by accelerating rate calorimetry. The results showed that Al or Mg substitution has little effect on improving the thermal stability of Li1.05Mn1.95-zMzO4. This is in contrast to the layered lithium transition-metal oxides (e.g., Li[Co1-zAlz]O-2, Li[Ni1-zAlz]O-2, Li[Ni0.8Co0.2-zAlz]O-2, and Li[Ni1/3Mn1/3Co1/3-zAlz]O-2), where Al substitution significantly lowers the kinetics of the reactions of the charged electrode materials with electrolyte. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3425606] All rights reserved.