Gas barrier properties of alkylsulfonylmethyl-substituted poly(oxyalkylene)s are discussed. Oxygen permeability coefficients of three methylsulfonylmethylsubstituted poly(oxyalkylene)s, poly[oxy(methylsulfonylmethyl)ethylene] (MSE), poly[oxy(methylsulfonylmethyl)ethylene-co-oxyethylene] (MSEE), and poly[oxy-2,2bis (methylsulfonylmethyl)trimethylene oxide] (MST) were measured. MSEE, which has the most flexible backbone of the three polymers, had an oxygen permeability coefficient at 30ΠC of 0.0036 1 10 013 cm 3 (STP)rcm/cm 2 rsrPa higher than that of MSE, 0.0014 1 10 013 cm 3 (STP)rcm/cm 2 rsrPa, because the former polymer's T g was near room temperature. MST with two polar groups per repeat unit and the highest T g showed the highest oxygen permeability, 0.013 1 10 013 cm 3 (STP)rcm/cm 2 rsrPa, among the three polymers, probably because steric hindrance between the side chains made the chain packing inefficient. As the side chain length of poly[oxy(alkylsulfonylmethyl)ethylene] increased, T g and density decreased and the oxygen permeability coefficients increased. The oxygen permeability coefficient of MSE at high humidity (84% relative humidity) was seven times higher than when it was dry because absorbed water lowered its T g . At 100% relative humidity MSE equilibrated to a T g of 15ΠC after 2 weeks. A 50/50 blend of MSE/MST had oxygen barrier properties better than the individual polymers (O 2 permeability coefficient is 0.0007 1 10 013 cm 3 (STP)rcm/ cm 2 rsrPa), lower than most commercial high barrier polymers. At 100% relative humidity, it equilibrated to a T g of 42ΠC, well above room temperature. These are polymer systems with high gas barrier properties under both dry and wet conditions. α§ 1998