The dependence with time of the permeated gases in the cavity of a probe equipped with a membrane made of a plurality of polytetrafluoroethylene (PTFE) capillary tubes was studied under a gas/membrane/gas configuration by performing tests at predetermined intervals of time in a range of 0-40 degreesC. The tests carried out at atmospheric pressure on both sides of the tubes with a multicomponent gas in the shell side of very low H-2, O-2, N-2, CO, CH4, CO2, C2H2, C2H4, C2H6 and C3H8 partial pressures in argon were used to establish the permeability coefficients (P-i), the permeation apparent activation energies (E-p) and the time required by each species to reach 99% of the equilibrium concentration (t(99)). At room temperature, the permeability coefficients computed from the slope of the steady-state transmission rate portion of the exponential growth function fitting the experimental data points showed the highest value for H-2 (12.1 Barrer) and the lowest value for C3H8 (0.27 Barter), with the other species inserted in between in the following order: P-H2 > P-CO2 > P-O2 > P-N2 > P-CO > P-C2H2 > P-CH4 > P-C2H4 > P-C2H6 > P-C3H8. These coefficients were shown to respond to an activated process (Arrhenius relation) and to be related to the molecular parameters of the penetrants (sigma(LJ)) except for CO2, where transport seems to be strongly influenced by some intermolecular interactions with the membrane material. The presence of such interactions is confirmed by the low E-p obtained for this molecule (4.93 kcal mol(-1)), and is supported by data reported in the literature for other membrane materials. Finally, the values obtained for t(99) showed that at least 40 h is required to get a representative sample (excluding C3H8) in the probe cavity when maintained at 40degreesC. This time lag was shown to double for a device submerged in oil due to the occurrence of concentration polarization phenomena at the oil/membrane interface. (C) 2004 Elsevier B.V. All rights reserved.