Isovalent substitution of Mn(II) for Mg(II) in the oxide ion conductor, La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85, yields a new oxide ion conductor which exhibits an ionic conductivity (σ = 4.6 × 10^–2 S cm^–1 at 800 °C) that is comparable to the conductivity of the Mg(II) analogue; interestingly, the Mn(II) oxide has a lower activation energy for conduction (E_a = 0.47 eV) than the Mg(II) oxide.