The dissolution of divalent cations plays an important role in the enhancement of conductivity for perovskite-based LaAlO3 and LaGaO3 oxygen conductors. In the LaAlO3 system, the solubility of MgO was less than 10% due to the mismatch of ionic radius between the Mg and Al cations. The substitution of Sr ions in the La-cation sublattice was as high as 20%. With the doubly-doping of SrO and MgO in LaAlO3, the enhancement of MgO solubility was also observed. However, further addition of MgO tends to lower the solubility of Sr ion from 20 to 10%. This result can be rationalized by the reduced distance between Mg ion and Sr ion that caused the cation-cation repulsion in the perovskite structure. In the singly-doped LaGaO3 systems, the solubility of MgO was found to be 20%. However, only less than 10% of the La-cation sublattice could be substituted by Sr ions. With the doubly-doping of SrO and MgO, the solubility of SrO was significantly enhanced by the addition of MgO. It is believed that the solubility enhancement of SrO is due to the expanded lattice caused by the addition of MgO. Within the solubility limit of the aliovalent cations, the ionic conductivities of both LaAlO3 and LaGaO3 systems increased with the increasing concentration of foreign cations. After the solubility limits in both doped LaAI03 and LaGaO3 were reached, the segregation of the second phase tends to lower the ionic conductivity drastically. The activation energy for ionic conduction was also dependent on the ionic radius of foreign cations which may affect the space available for the transport of oxygen ions. (C) 2004 Elsevier B.V. All rights reserved.