The phase stability of Sr-doped LaMnO3 (LSM) perovskite in the La-Sr-Mn-O system was investigated as a function of Sr content and A/B cation ratio. The perovskite structure changed with both Sr content and A/B cation ratio. As the Sr content was increased to 0.2 mol, the perovskite structure adopted an orthorhombic distortion. This changed to a monoclinic or hexagonal structure for 0.2 less than or equal to Sr less than or equal to 0.3 mol. When the Sr content increased to 0.3 mol, the structure reverted to orthorhombic symmetry. Data from X-ray powder diffractometry, scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray emission spectroscopy, showed that secondary phases in the La-Sr-Mn-O system were underdeveloped with respect to changes of the doped Sr content, A/B cation ratio, and thermal history. X-ray detectable minor phases present in the LSM material were Mn3O4, (La, Sr)Mn2O4, La2O3, La(OH)(3), (La, Sr)(3)Mn2O7, and (La, Sr)(2)MnO4.