All manual controls of Fox's retarding potential difference (RPD) technique have been replaced by automatic electronic controls applied to a commercial time of flight mass spectrometer. The current difference &Dgr;Ibetween the electron energy voltage increments, is obtained from an electronic data processor and plotted as a function of the ionizing voltage. An ionization efficiency (IE) curve is obtained in 3.8 sec or less. The sensitivity of the system permits an IE curve to be obtained in the 10−8Torr range. An empirical formula was required to correct the present systematic error. The error was found to be −3.67+0.834x−0.0306x2for xenon, krypton, and argon, wherexis the ionizing voltage of the data. This formula was used to correct the experimental water ionization potential (IP) to give 12.59±0.05 V. The spectroscopic water IP is 12.61 V.