Doping effects of Al on crystal structures and electrical conductivities of LaGa0.9Mg0.1O2.95 perovskite compounds were investigated. As the amount of doping with Al increased, the symmetry of structure changed to the orthorhombic -> the rhombohedral -> the cubic, which is reportedly favorable for oxide ion conduction, and lattice parameters decreased monotonically. The Al-doped LaGa0.9Mg0.1O2.95 compounds showed mixed (hole and ion) conduction in air and pure oxide ion conduction in low oxygen partial pressure. The oxide ion conductivities decreased with Al contents in spite of the cubic structure formation. The degradation in oxide ion conduction was attributable to narrowed oxide ion conduction paths in Al-doped structures. (c) 2007 Elsevier B.V All rights reserved.