The solid solution range of the La(Sr)Ga(Mg)O-3 perovskite was investigated and the oxygen ion conductivities of corresponding compositions were measured. The solubility of strontium could be extended by the addition of magnesium ions to establish a balance in the oxygen vacancies associated with A and B sites. Ln this way, the Sr limit in the solid solution was increased to more than 20% of the La content. Doping with either Sr or Mg enhanced oxygen ion conductivity, but whereas Sr doping yielded decreased activation energy with increasing concentration, Rig produced a higher activation energy with increased Mg content. La0.8Sr0.2Ga0.9Mg0.1O2.85 was found to have the highest conductivity apart from CeO2-based electrolytes at temperatures below 700 degrees C, while La0.8Sr0.2Ga0.85Mg0.15O2.825 has the highest conductivity except for Bi2O3 at temperatures above 700 degrees C.