Densification of La0.90Sr0.10Ga0.80Mg0.20O3-delta (LSGM) has been achieved by applying an isostatic pressure using HIP. Pore free LSGM with 99.6% in relative density was obtained by HIPing at 1300 degrees C for 1 h under 200 MPa. The ionic conductivities of HIPed LSGM were confirmed to be comparable to those of low density LSGM obtained by conventional sintering process. A single phase of LSGM containing high magnesium content La0.90Sr0.10Ga0.70Mg0.30O3-delta, which had not been obtained under atmospheric pressure, was also synthesized by applying a HIPing pressure. The ionic conductivities were higher than those of HIPed LSGM with low content of magnesium. It was confirmed that the transport numbers of oxygen ions in the LSGMs were almost unity. (c) 2006 Elsevier B.V. All rights reserved.