We investigated the effect of mechanical deformation on transport properties by deforming synthetic calcite/quartz aggregates to strains (ϵ) up to 5% at confining pressures (Pc) up to 300 MPa, and at temperatures (T) from 300 K to 873 K. Subsequently, we measured permeability (k) using a wide-range permeameter at effective pressures (Pc) of 15--155 MPa and room temperature. We then measured electrical conductivity (&sgr;) in an impedance-measuring assembly over the same pressure range at room temperature. The values of permeability and conductivity of the undeformed material at Pc=100 MPa were 4¿10-18 m2 and 0.07 S/m. Samples deformed at 673 K and Pc=200 MPa, or at room temperature and Pc=50--300 MPa, show small variations in permeability and conductivity: k changed only by up to a factor of 3 and &sgr; increased by up to 10%. But, when a sample was deformed at 873 K and Pc=200 MPa, electrical conductivity dropped by 1 order of magnitude and permeability dropped by 2 orders of magnitude. To assess whether changes in length scales of the pore structure owing to deformation may account for large variation in transport properties, we counted cracks and pores, measured their lengths and widths, defined a damage parameter (&xgr;), and computed effective hydraulic and electrical conductivity using renormalization group methods. The undeformed rocks and the samples deformed at low confining pressure have severe damage, appear to be close to failure, and hence have high transport coefficients. Materials deformed at high pressures and temperatures have lower flaw densities, connectivities, and transport coefficients after deformation. We found that renormalization methods are suitable to model connectivity loss and large changes in transport properties owing to changes in flaw density and length scales. Pore connectivity and transport properties vary strongly during semibrittle deformation.¿ 1997 American Geophysical Union