Permeability coefficients and ideal selectivities for H-2, CO2, O-2, and N-2 were determined for two series of polyimides with indan groups in the main chain. The bulkiness of the indan group was varied by replacing methyl groups at the indan ring system by bulky cyclohexyl groups. The effects of the linking group in the dianhydride part of the polymer repeating unit could be explained on the basis of bulkiness and flexibility. However, the introduction of two cyclohexyl substituents at the indan group to increase the bulkiness of this structural element had no uniform effect on permeability coefficients and selectivities. Positive or negative effects were observed, depending on the linking group in the dianhydride moiety. This was tentatively attributed to the conformational flexibility of the cyclohexyl rings, which can result in different sterical requirements.