series of polyalkylenebenzimidazole were synthesized by solution polycondensation and polymeric membranes were cast from formic acid solution at low pressure and room temperature. The relationship between gas permselectivity and chemical structure of polyalkylenebenzimidazole was discussed.It was found that the permeability order of hydrogen;carbon dioxide;oxygen;nitrogen;and methane is fit for ordinary glass polymers.The polymers that have longer flexible polymeric chain have higher gas permeability

but lower selectivity than the polymer that have shorter flexible chain. Gas permeability rises and selectivity reduces if temperature of transport gas increases. It reveals that the gas permeability will rise

if flexible segment is introduced in to rigid backbone for some aromatic polymers.