Ionization measurements of water vapor in vapor-liquid equilibrium were made with a series of chambers on a tritiated water sample which was prepared by burning T2 to T2O and diluting it with H2O. A correction was made for the difference of vapor pressures of HTO and H2O. The effect of HTO adsorbed on chamber walls and electrodes was eliminated from the final result. Using Gerbes data on the number of electron volts to produce an ion pair in air, agreement obtained between the measured source strength and the source strength as computed from pressure, temperature, volume, purity, and dilution data on the original gaseous T2 sample.