Changes in Q in rocks in response to thermal cycling can result from the removal of physisorbed water, an increase in the crack density and therfore the internal surface area, at temperatures greater than 400 ¿C, the removal of chemisorbed OH groups which are the adsorption sites for physisorbed water, and the dehydration of clays. When rocks are heated to temperatures of 100¿-200 ¿C, the change is Q is reversible and is due to the removal of physisorbed water. Sioux quartzite and Berea sandstone were thermally cycled to 657 ¿C and 385 ¿C, respectively. The Q of both rocks increased when measured over a broad range of relative humidities, but decreased at low humidities (<10--25%). Measurements of adsorption isotherms for the Berea sandstone showed no loss of adsorptive capacity of the clays. Although the surface area of thermally cracked rocks increased, the number of sites where adsorbed water molecules contribute to attenuation decreased.