<,正,>, The effects of chemical structure, i. e. side chain structure and their contents, on thepermeability of pure SO,2, N,2, and their mixture gases for the sulfoxide grafted poly （vinylalcohol） （RVSO-PVA） membranes have been investigated:where R=Me, Et, Pr, t-Bu and Ph. It was notable that introduction of sulfoxide group intoPVA side chain greatly enhanced the permselectivity of sulfur dioxide. SO,2, permeability andseparation factor of these polymers increased markedly as the size of side chain increased. Thesulfoxide content of the polymer also played an important role in the pure and mixture gasespermeation. Some explanations have been made to interpret this unique gas separation behaviour.