The performance of a compacted soil liner is partly a function of the porosity, which is important because the transport of materials through the liner occurs via the pore space. This project studies the pore spaces of compacted soil materials to estimate the effective porosity, which is the portion of the pore space where the most rapid transport of leachate occurs. Pore space of three soil materials, till, loess. and paleosol, was studied by using mercury intrusion porosimetry, water absorption, and image analysis. These analyses provided cumulative porosity curves from which the pore size distribution of soil samples were estimated. Theory was developed to estimate the effective porosity of a compacted soil material based upon a model of its pore size distribution and pore continuity. The effective porosities of compacted till. loess. and paleosol materials are estimated-to be 0.04. 0.08. and 0.09. respectively. These values are 10 to 20% of the total porosities. Comparisons between measured and predicted C1 travel times through compacted soil samples were made in order to verify the estimated effective porosities. The estimated effective porosities are reasonable because predicted C1~ first breakthrough times are similar to the measured first breakthrough times in the,soils studied. For these three soils predicted first breakthrough times are 5 to 10 times earlier when effective porosity is used.in the Darcy-equation based calculations as compared to Darcy-equation-based calculations that utilize total porosity.