A modified reconstruction algorithm has been presented for generating a three-dimensional microstructure from a single two-dimensional image of an isotropic porous medium based on overlapping spheres. For these model systems, the algorithm was observed to reproduce visual characteristics of the original systems for porosities ranging from 15-40%. The reconstruction algorithms have also been evaluated based on their ability to reproduce the transport properties of the original systems. For porosities greater than 25%, both the average relative conductivity and permeability of the modified reconstructed systems were within a factor of 2.7 of the values for the original system. For porosities greater than 30%, the average permeabilities of the modified systems were within 25% of the permeabilities of the original microstructures. These improvements in permeability agreement have been related to the critical diameter measured for the microstructures. For lower porosity (<20%) systems, the modification algorithm had little effect on this critical diameter and thus little improvement was observed in the permeability values of the modified reconstructed systems. All reconstructed systems appear to exhibit a pore space percolation threshold of 8%, significantly higher than the 3% value for the original penetrable sphere system, which further contributes to the disagreement in transport properties for the lower porosity systems.