A novel permeation apparatus was developed for the selective on-line measurements of gas fluxes. The development of the apparatus was motivated by the need to measure transient fluxes, directly, rapidly, precisely and selectively. To achieve these goals, a residual gas analyzer was used as a selective pressure transducer. Lag-time experiments allowed the simultaneous determination of permeation characteristics, such as permeability and diffusion coefficients. These coefficients are constant within the studied feed pressure interval (0.1-0.0 1 atm at 28 degrees C). Pure nitrogen, helium, carbon dioxide, methane and formaldehyde permeation was performed through polydimethylsiloxane, polyisoprene, polyoctenamer, and polyurethane membranes. Classical transport properties were very similar to literature values. The highest permeability, diffusivity and solubility values are found for the polydimethylsiloxane membranes. Carbon dioxide presents the lowest diffusivity and the largest solubility in all membranes, and formaldehyde permeation behaviour is similar to that of carbon dioxide. Physical properties of penetrants (critical volume and normal boiling temperature) and of membranes (glassy transition temperature) are good indicators of the permeation behaviour. This study documents a novel permeation approach to allow precise on-line transient fluxes to be quickly and directly measured. (c) 2006 Elsevier B.V. All rights reserved.