Thick composites (~ 3 mm in thickness) of polypyrrole with electrically insulating porous (polystyrene) and nonporous (polymethyl methacrylate) substrates were prepared using a two-step batch method. In the two-step method, impregnation of volatile (iodine) or nonvolatile (ferric chloride) oxidant in the substrate is followed by in-situ polymerization of pyrrole. Conductivities as high as 10-1 S/cm were obtained in this work in the case of composites of polypyrrole and porous, crosslinked polystyre. Use of the nonvolatile oxidant (ferric chloride) resulted in higher conducting polymer yield, as well as composites having a higher conductivity, thermal stability, and mechanical strength. However, the volatile oxidant (iodine) could be transported to the substrate using supercritical carbon dioxide as the solvent. As a result, partitioning of the oxidant between the solvent phase and the polymer substrate, and hence the distribution of the oxidant in the substrate, could be controlled by manipulation of the pressure. The two-step batch method in which supercritical carbon dioxide is used to facilitate transport and as a solvent for the oxidant was found to be an effective method for the production of thick composites with uniform conductivity, thermal stability, and mechanical strength. Such composites are desired in important practical applications such as rechargeable battery electrodes and electromagnetic interference shielding materials.