Sputtered iridium oxide films (SIROF) are investigated as a pH sensitive membrane to be used in aqueous solutions in wide range of temperature and pressure. The temperature dependence of the pH response of SIROF is almost equal to the Nernstian theoretical value of 2.303 kT/q in the temperature range between 0–100 °C. The long term stability of the pH response and the influence of various ion interferences seems to be in an acceptable range for practical use. SIROF is also found to be stable in high temperature and high pressure water up to 200°C and 4000 psi. By usiing SIROF as an ion sensitive membrane combined with a metal silicide coating as a barrier layer, the ICD (ion controlled diode) is shown to be a promising candidate as a high temperature and high pressure pH sensor.