In order to develop a pH sensor having a wide pH response range from strong acid to strong alkali, electrolyte-insulator-semiconductor capacitors using corrosion-resistant Al2O3-Ta2O5 and Al2O3-ZrO2 double-oxide films as the insulator were fabricated and their response characteristics to H+ and alkali metal ions were examined. The sensors with Al2O3-Ta2O5 and Al2O3-ZrO2 films having an Al3+ cationic fraction of about 0.4 showed a wide linear pH response range of -1.5 to 13.0 and a small alkali metal ion sensitivity. The use of the Al2O3-Ta2O5 film is suitable for increasing pH sensitivity and that of the Al2O3-ZrO2 film is promising for increasing corrosion resistance against strong alkali solutions. (C) 2004 The Electrochemical Society.