The correlation between the porous structure of electrodes designed for double-layer supercapacitors and the series resistances of the latter is addressed in this paper. It is shown that in such composite films, made of active carbon embedded in a porous polymer, the internal electrolyte resistance within the pores governs the series resistance. In order to evidence this, various compositions of polymer-carbon mixtures are manufactured. Their porosities are determined, both before and after impregnating them with the solvent of the electrolyte. For the compositions under study, the nature of the solvent is found to induce very different effects on the permeability of the films. However, the transport properties within the pore space of the composites are also investigated via ion diffusion experiments. Whatever the solvent, a strong correlation is found between formation factors of the porous films and the series resistances of the final double-layer supercapacitors.