Silica is an important component of thermal insulation used in aerospace transport vehicles. Although normally hydrophilic and requiring rewaterproofing after flight, silica insulation has been reported to become hydrophobic on heating and cooling in a vacuum and, therefore, potentially hydrophobic after reentry. To understand and model these processes, the rates of desorption and adsorption of water on silica were studied by thermogravimetry, Fourier transform infrared spectrometry, and X-ray photoelectron spectroscopy. Silica powders with a high surface area were heated in vacuo in the temperature range 200-1000 degrees C, infrared absorbance spectra were obtained at various temperatures, and surface structure was examined by X-ray photoelectron spectroscopy. A kinetic sorption model was developed based on a Langmuir rate expression. Activation energies for adsorption and desorption were determined by fitting the experimental results numerically. The study found adsorption of water on the heat-treated silica occurs within seconds, implying hydrophobicity is lost rapidly after the heat-treated silica is exposed to water.