Traditional techniques for measuring permeability and traditional models relating permeability to porosity are inadequate for low-permeability (≤10-18 m2) geologic materials. Special techniques have been developed for measuring fluid flow through such rock samples and for quantifying their three-dimensional pore microstructure. Permeability is measured by a modified transient pulse technique, pore structure is quantified using a computerized microgeometric modeling system, and portable three-dimensional display is produced by an optical imaging system. Results of applying this processing to a low-porosity ''tight gas sandstone'' from the Cotton Valley Formation agree with the well-documented diagenetic history of these samples. We find that the porosity distribution is very inhomogeneous and skewed. The pore system is slightly anisotropic, which is consistent with the differences in permeability measured in two orthogonal directions. We indicate how such data could be used to develop more geologically and geometrically realistic models relating porosity and permeability in low-permeability gas formations.