CO2 transport through functional assembled mono-layers was evaluated in relation to H2O and nonpolar gases such as CH4, O-2, N-2. Membranes based on Pebax (R) 2533 were functionalised by incorporating chemical compounds containing free hydroxyl, N-alkyl sulphonamide, bulky benzoate groups. The effects of both the chemical nature and concentration of the modifier on the gas transport were reported, respectively. The permeability coefficients of different penetrating chemical species were compared, evidencing the higher affinity of the layers to water vapour and carbon dioxide, due to favourable interactions between polar moieties and penetrant. The condensability of the penetrant directed the permeability of the species considered and was responsible for the high solubility selectivity between H2O and CO2 (i.e. P-H2O = 20, 330 barrer, D-H2O/D-CO2 = 0.6, S-H2O/S-CO2 = 11.4 at 25 degrees C for Pebax/KET 50/50 w/w). An increase in polar moieties resulted in enhanced permeability and selectivity with respect to the pure polymer. In contrast, the functionalised polymer was not capable to discriminate between the smallest penetrants such as O-2 and N-2, with consequent decrease in the ideal selectivity (P-CO2/O2, P-CO2/N2). The functional layers exhibited permeability and selectivity covering broad ranges of values. (c) 2005 Elsevier Ltd. All rights reserved.