The gas permeation behaviour of three gases (N-2, O-2 and CO2) were studied through three different hexafluoroisopropylidene (6FDA) based novel poly(imide-siloxane) (PIS) membranes. The rigidity of the imide groups were varied by utilizing three different amines namely the flexible siloxane containing amine, diaminodiphenyl ether and diaminodiphenyl sulphone. The permeability value was found to be maximum for the more flexible siloxane containing PIS and there is noticeable reduction in permeability with the more rigid sulphone imides. A maximum reduction in the permeability of 600 Barrer was observed for CO2 gas with sulphone imide containing PIS when compared to PDMS that showed CO2 permeability of 2200 Barrer. The effect of pressure and temperature on the permeability of these gases was examined. The effect of glass transition temperature and the fractional free volume on the permeability parameters was discussed. The relationship between the structural rigidity of the membranes and the gas transport properties were established. The maximum selectivity for O-2/N-2 gas pair was obtained with the sulphone imide containing PIS, whereas the highest selectivity of 6.8 was achieved with aromatic ether imide containing PIS for CO2/N-2 gas pair. (C) 2007 Elsevier B.V. All rights reserved.