The monoclinic and orthorhombic ordered-rocksalt polymorphs of LiMnO2 are of interest as high-capacity cathode materials for rechargeable Li batteries. The relative stability of the monoclinic and orthorhombic phases has been studied as a function of firing temperature, oxygen partial pressure, and Al doping. Control of temperature and oxygen partial pressure is crucial in order to obtain the LiAlyMn1-yO2 phases while minimizing impurity phases such as LiMn2O4, Li2MnO3, and Mn3O4. The solid solubility of Al in LiMnO2 has been determined to reach a maximum of 5-7% at 950 degrees C and P-O2 = 10(-7) atm. The existence of a maximum in Al solid solubility is attributed to the competing stability of gamma-LiAlO2 at higher temperatures. A phase stability mechanism for the monoclinic phase upon Al-doping is proposed.