Degussa Aerosil TK 800 was thermally dehydrated in vacuo in the temperature range 20 to 900°C. The accompanying changes in water content, number of silanol groups, specific surface area from methanol adsorption and heat of immersion in methanol were determined. From adsorption measurements, S BET Me areas are smaller than S BET N2 areas ( J. Colloid Interface Sci., in press). The silanol groups, acting as primary centers for methanol adsorption, make methanol adsorption a measure of the hydrophilic surfaces which represent the domains of surface hydroxyls. In addition, these hydroxyls appear to display a significant role in controlling the values of the heat of immersion in methanol up to 380°C. Surface methoxylation of strained siloxane bridges seems to play an influencing role over the range 380 to 510°C, after which both surface depletion of hydroxyls and an increased resistance to reaction of annealed siloxane bridges contribute to a decrease in the immersional heat value at 900°C. In general, the integral heat of immersion in methanol of a silica surface is considered to comprise energetic contributions from such processes as H-bonding of methanol to silanol groups, methoxylation of siloxane bridges which became annealed to varying extents depending upon the treatment temperature, in addition to the participation of dispersion forces.