High molecular weight metathesis poly(5-ethylidene-2-norbornene) (PENB) has been synthesized in the presence of the 1st generation Grubbs catalyst at a high monomer/catalyst ratio (3000/1 and higher). The yields of the corresponding polymer have been more than 90% at all monomer/catalyst ratios and molecular weights (M (w) ) of PENB have been higher than 3 x 10(5). The gas permeability of different gases (He, H-2, O-2, N-2, CO2, and CH4) through PENB films has been studied, and diffusivity and selectivity coefficients have been determined. It has been found that PENB is more permeable than unsubstituted metathesis polynorbornene and cycloalkyl-substituted polynorbornene dicarboximides, but it is less permeable than some Si-containing polynorbornenes. Despite the fact that PENB is more permeable than metathesis polynorbornene and cycloalkyl-substituted polynorbornene dicarboximides, it is close to these polymers in ideal selectivities for some gas pairs.