The transport-reaction mechanism taking place in porous cathodes used in lithium-sulfur battery cells is systematically investigated using impedance spectroscopy. The adverse effects of metallic lithium are excluded by using symmetrical carbon-carbon cells. The results obtained on two types of porous electrodes, a carbon felt and a conventional mesoporous carbon electrode, are compared to those previously obtained on a flat glassy carbon electrode. The thickness of both porous electrodes as well as the thickness of porous separator are varied and impedance spectra studied in detail. The shapes, magnitudes and trends of various impedance features could well be reproduced by a proposed transmission line model. The model is upgraded with respect to known transmission line models in two ways: diffusion in pores of electrode is taken into account and a coupling of this diffusion to diffusion in separator is carried out. The results consistently show that the migration, reaction and diffusion impedances due to processes taking place inside a typical cathode are one to two orders of magnitude smaller than the impedance due to diffusion of active species in the porous separator. However, the present findings strongly indicate that if one could suppress the leakage of polysulfides into the separator, the total impedance would drop dramatically, regardless of the actual thickness or porosity of the cathode. (C) 2019 Elsevier Ltd. All rights reserved.