Phosphosilicate (P2O5-SiO2) gels with an addition of Al2O3 or B2O3 as a third component have been prepared by the sol-gel method aiming at the improvement in chemical durability of the gels. The presence of strongly interacted water with gel matrices was confirmed for all the gels from an endothermic peak with weight loss at around 230 degreesC in thermal analyses. The addition of Al2O3 or B2O3 suppressed the formation of Si5O(PO4)(6) in phosphosilicate gels during heat treatment and Al2O3 remarkably improved the chemical durability of the resultant gels after a heat treatment at 300 degreesC. Al2O3-added gels (P/Si/ Al = 1:0.9:0.1 molar ratio) showed conductivities of about 10(-4) Scm(-1) at 130 degreesC and 0.7% R.H. after a pre-treatment at 30 degreesC and 60% R.H. for 3 h, whereas pure phosphosilicate gels (P/Si = 1: 1) and B2O3-added gels (P/Si/B = 1:0.9:0.1 molar ratio) showed higher conductivities of about 10(-2) Scm(-1) under the same conditions. The lower conductivity of the Al2O3-added gel was ascribed to the smaller amounts of H3PO4 formed by hydrolysis Of Si5O(PO4)(6) in the gels during the pre-treatment. (C) 2003 Elsevier B.V. All rights reserved.