The synthesis and polymerization of the new monomers N-(1-adamantyl)-exo-norbornene-5,6-dicarboximide (AdNDI), N-cyclohexyl-exo-norbornene-5,6-dicarboximide (ChNDI), and N-phenyl-exonorbornene-5,6-dicarboximide (PhNDI) are reported. Copolymers of IV-(1-adamantyl)-exo-norbornene-5,6-dicarboximide/norbornene, with molar compositions 50/50, 70/30, and 30/70, were also obtained. The transport of hydrogen, oxygen, nitrogen, carbon monoxide, carbon dioxide, methane, ethylene, and ethane across membranes prepared from these homopolymers and copolymers was determined at 30 degreesC using permeation techniques. Diffusion coefficients correlate rather well with the diameter of diffusant molecules except in the case of carbon dioxide. The values of the permselectivity coefficient for different gases depend on the type of membranes. For example, the permselectivity of oxygen with respect to nitrogen, alpha(O-2/N-2), is ca. 5.50 for membranes prepared from N-(1-adamantyl)-exo-norbornene-5,6-dicarboximide/norbornene (50/50) copolymers, which compares favorably with the values reported for this parameter in membranes with imide groups located in the backbone. The values of alpha(H-2/C2H6) in membranes of poly(N-(1-adamantyl)-exo-norbornene-5,6-dicarboximide), poly(N-cyclohexyl-exo-norbornene-5,6-dicarboximide), and poly(N-phenyl-exo-norbornene-5,6-dicarboximide) are 183, 124, and 122, respectively, whereas the values of alpha(C2H4/C2H6) amount to 6.9, 7.6, and 6.4, respectively. For most membranes used in this study, diffusion rather than solubility is responsible for the discrimination of gas transport. However, solubility is mainly responsible for the high permselectivity of ethylene with respect to ethane displayed by the membranes.