Several polyimides were studied with regard to the influence of their conformational rigidity on the packing in glassy state, and consequently on their physical properties such as glass transition temperature and selectivity of gas separation membranes made from these polymers. The values of their physical properties were taken from literature, while the conformational rigidity parameters such as Kuhn segment, characteristic ratio, and occupied, free, and accessible volumes were calculated here and were correlated with physical properties. It was shown that there are correlations between selectivity of gas separation membranes made from these polyimides on one hand, and characteristic ratio, on another hand.