Connected porosity volume and throat diameter were measured in granitic samples with different porosity structures heated from 20 to 700 ¿C. Three types of microstructures are recognized from SEM analysis: (1) a crack network of transgranular cracks with few intersections (2) a pipe network of intragranular and intergranular tubes with high connectivity (3) a mixed structure of transgranular cracks connected by intragranular tube structures. Calculations predict a decrease of permeability through crack and mixed networks between 50 and 400 ¿C followed by a sharp increase at higher temperatures. The decrease in permeability is associated predominantly with the decrease in throat diameter, whereas the increase in both throat diameter and the porosity volume account for the temperature rise. Only small variations of the permeability are predicted for the pipe network. ¿ American Geophysical Union 1994