Consistently lower tritium concentrations found in water distilled from silica gel than in the water vapor that had been adsorbed from air stimulated this study of isotopic exchange and the vapor pressure isotope effect for tritiated relative to nontritiated water. Tritium concentrations were compared for various amounts of water vapor adsorbed on and desorbed from a fixed amount of silica gel dried at 100 degrees C. Tests of the hypothesis that a constant percent water and hydroxyl groups bound in the silica gel undergo isotopic exchange with the adsorbed tritiated water were performed with a type of silica gel generally used in ambient air monitoring. Separate tests were performed to evaluate the additional impact of the vapor pressure isotope effect in reducing the tritium concentration in water vapor relative to the condensed phase. This occurs (1) in the tests when air is swept through tritiated water to generate airborne HTO for adsorption on the silica gel and (2) in distilling water adsorbed on silica gel for tritium analysis. A value for exchangeable water on the silica gel of 5.9 +/- 0.2% was found when adsorbing between 7 and 22% water. The isotope effect reduced the tritium concentration in air relative to water by 11% at 20 to 22 degrees C and by 4% in azeotropic distillation at 80 degrees C.