Infrared spectroscopy has been used to show that the surface hydroxyl groups on highly dehydrated silica will exchange with 18O-labeled water. The degree of exchange depends on the sample temperature and is a maximum at about 400 °C, where approximately 65% of the silanol groups contain 18O. The use of the 18O exchange technique for assigning surface-adsorbate stretching modes is illustrated by considering the infrared spectra of BF3 chemisorbed on silica.