The rehydroxylation reaction of siloxane bridges was studied for silica gel particles dehydrated by preheating. Adsorption isotherms of water vapor at 298 K were measured for the samples which were preheated at different temperatures. The amounts of total adsorption and irreversible adsorption were examined against the preheating temperature and vapor pressure. The number of the hydroxyl groups produced by the rehydroxylation of siloxane was measured by thermogravimetrican alysis to determine the adsorption isotherms for chemisorption of water. For the surface preheated at 673 K, the siloxane bridges were fully rehydroxylated for about 3 h at a relative pressure of 0.3. The surface preheatedat 873 K was gradually rehydroxylated with increasing pressure and the rehydroxylation was not complete even for 90 h at a relative pressure lower than 0.7. In the high-pressure region, the rehydroxylation was promoted by condensed water. The number of surface hydroxyls after rehydroxylation became greater than that on the original surface. The possibility of the generation of the geminal hydroxyls was suggested from the degree of rehydroxylation on the surface.