The transport of hydrogen, carbon dioxide and nitrogen through a microporous tubular silica membrane has been investigated between 20 and 200 degreesC and 3-9 bar upstream pressure. Pure compounds permeabilities decrease from H-2 to N-2 and do not show a strong dependence upon upstream pressure. Temperature variation could be described by an Arrhenius law with low apparent activation energies (3.5, 3.7 and 3.4 kJ mol(-1), respectively, for hydrogen, carbon dioxide and nitrogen). The ideal separation selectivity computed from these results leads to values around 3.5 and 3 for H-2/CO2 and CO2/N-2 Separation independent of temperature. These values are significantly smaller than those expected from a strict Knudsen mechanism (4.7 and 3.7, respectively). A viscous contribution, resulting for instance from a too large pore size distribution of the active silica layer, possibly accounts for the experimental results obtained.