Packing density and gas permeability of a three-component copolyimide series is presented. The three-component polyimides are prepared a via "stepwise" synthesis procedure that goes through the acid anhydride terminated pre-polymer. The procedure ensures the statistical distribution of segments of the polymers. The polyimide series is composed of contrasting segments : a bulky and rigid hexafluoroisopropylidene-2,2-bis(phthalic acid anhydride)/9,9,-bis(4-aminophenyl)fluorene and a flexible hexafluoroisopropylidene-2-2-bis(phthalic acid anhydride)/2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane,with varying segment ratio. Generation of additional free volume by copolymerizing two segments is observed. The permeability of six pure gases - He, H-2, N2, O2, CH4, and CO2 - to the polyimides showed positive deviation from the simple additivity rule of segment weight ratio reflecting the generation of free volume. However, a conflicting result between free volume fraction and gas permeability is observed, which may be due to a difference of the nature of free volume of each segment.