In order to develop an insulator film for an electrolyte-insulator-semiconductor capacitor pH sensor which can be used in a wide pH range, the corrosion resistance of Al2O3-Ta2O5-ZrO2 films formed by metallorganic chemical vapor deposition has been investigated. Dissolution rates of the films deposited on a Pt substrate were measured by ellipsometry in 6 M HCl and 1 M NaOH solutions. The films with the cationic mole fraction of Al, X-Al, smaller than 0.4, the cationic mole fraction of Ta, X-Ta, larger than 0.3, and the cationic mole fraction of Zr, X-Zr, larger than 0.3 showed high corrosion resistance against both solutions. (c) 2005 The Electrochemical Society. [DOI: 10.1149/ 1.1945607] All rights reserved.