The crystal structures, microstructures and electrochemical properties of Al-doped lithium manganese oxide materials LiAl x Mn1−x O2 (0 ≤ x ≤ 0.1) prepared by solid state reactions have been investigated. A1 doping results in increased cation disorder in the orthorhombic polymorph of LiMnO2, and produces layered monoclinic LiMnO2 with an α-NaFeO2 type crystal structure. The formation of monoclinic LiAl x Mn1-x O2 confirms earlier observations by Chiang et al. [1,2]. A mechanism is proposed for the orthorhombic-monoclinic transformation, based on Li-Mn inversion in the orthorhombic structure. Al ions substitute in Mn sites in the monoclinic phase and give rise to microstrain in the [2 0 -l] planes. Microstructural analysis by scanning electron microscopy has revealed Al-deficient striations which may represent residual zones of orthorhombic phase. In cycling tests in Li button cells, increasing the amount of Al dopant extends the number of cycles required for the capacity to evolve to its maximum value, but results in increased stability of the capacity at 55 °C. The layered structure of the monoclinic materials is retained on the first cycle, but transforms to a spinel-type structure on extended cycling.