The micropore structure of granite samples has been studied by analyzing the results of porosity, electrical and permeability measurements. The effective porosity, connecting porosity, tortuosity and permeability are calculated from these measurements; their values for these samples are in the order of 0.19-0.64%, 0.06-0.34%, 1.7-4.5 and 0.1-54.9 microdarcies, respectively. Alteration reduces the connecting porosity and permeability, and increases the tortuosity. This is explained by reduction of the aperture of pore pathways due to dissolution of certain minerals followed by deposition of secondary minerals. The results also suggest that laboratory data for permeability may be extended to field conditions below a 400-meter depth.--Modified journal abstract.