The gas transport properties of several simple vinyl and vinylidene polymers are compared to examine the effect of pendant group type (H, CH3, F, Cl) and symmetry of placement. The literature contains extensive information about these polymers, which has been critically reviewed to obtain consistent data for interpretation of the effect of structure on gas permeability. The gas transport data available for many of these polymers are quite variable because of differences in additives, thermal history, and crystallinity. The values used here are from studies where the physical properties of the film were reported along with the transport data. The appropriate data were not available for poly(vinyl fluoride) or poly(vinylidene chloride). For poly(vinyl fluoride), permeability measurements and thermal analysis were done to supply this information. Results for poly(vinylidene chloride) were obtained by extrapolation of copolymer permeation properties. Estimates of the permeability of oxygen in the amorphous phase of each polymer are discussed in terms of the estimated fractional free volume of that phase. In this way, the intrinsic effects of molecular structure on gas permeation exclusive of crystallinity effects have been evaluated.