Single-phase Li[M x Mn 2 - x ]O 4 (x=0.5 and M=Co, Al and Ti) spinel compounds were synthesized by solid-state reactions. The lattice parameter of Li[Co 0 . 5 Mn 1 . 5 ]O 4 , Li[Al 0 . 5 Mn 1 . 5 ]O 4 and Li[Li 0 . 0 4 Ti 0 . 4 9 Mn 1 . 4 7 ]O 4 was 0.8134, 0.8150, and 0.8288 nm, respectively. The change in the lattice parameter relative to Li[Mn 2 ]O 4 was found consistent with the modified concentrations of Mn 4 + ions upon Co 3 + , Al 3 + and Ti 4 + substitution. A reversible reaction at 5.1 V vs. lithium was observed for Li/Li[Co 0 . 5 Mn 1 . 5 ]O 4 cells, which was associated with the redox of Co 3 + /Co 4 + ions. The presence of an oxidation reaction at 5.1 V (partially reversible upon reduction at 4.9 V) in Li/Li[Al 0 . 5 Mn 1 . 5 ]O 4 cells suggested that the near 5 V electrochemical reaction could not be attributed solely to the redox of substituted transition metal ions in the lithium manganese spinel compounds. In contrast, Li/Li[Li 0 . 0 4 Ti 0 . 4 9 Mn 1 . 4 7 ]O 4 cells, similar to Li/Li[Mn 2 ]O 4 cells, did not show any reactions at voltages above 4.6 V vs. lithium. It is believed that increase in the Mn 4 + ion concentrations (or the Li/Mn 3 + ratio) in the spinel framework upon substitution (like Co 3 + and Al 3 + ) is essential for the presence of electrochemical reactions near 5 V in substituted lithium manganese spinel compounds. This hypothesis is consistent with the mechanism proposed by Ohzuku et al. (J. Power Sources, 1999;81/82:90-94), which is related to compact crystal fields imposed by the cubic close-packed oxygen arrays linked with tetravalent manganese ions in the spinel framework.