The gas-phase molecular-flow of hydrogen or helium through two well-characterized porous media containing adsorbed sulphur dioxide molecules has been examined. The permeabilities observed depend markedly on the adsorbate concentration, and it is possible to explain the variation semi-quantitatively in terms of the known pore structures of the porous media. It is suggested that similar experiments would often be useful in pore structure investigation, and the sort of information which could be obtained is examined.