Gas barrier properties of alkylsulfonylmethyl-substituted poly(oxyalkylene)s are discussed. Oxygen permeability coefficients of three methylsulfonylmethyl-substituted poly(oxyalkylene)s, poly[oxy(methylsulfonylmethyl)ethylene] (MSE), poly[oxy(methylsulfonylmethyl)ethylene-co-oxyethylene] (MSEE), and poly[oxy-2,2-bis (methylsulfonylmethyl)trimethylene oxide] (MST) were measured. MSEE, which has the most flexible backbone of the three polymers, had an oxygen permeability coefficient at 30 degrees C of 0.0036 x 10(-13) cm(3)(STP) . cm/cm(2) . s . Pa higher than that of MSE, 0.0014 X 10(-13) cm(3)(STP) . cm/cm(2) . s . Pa, 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 x 10(-13) cm(3)(STP) . cm/cm(2) . s . Pa, 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(alkylsulfonyl-methyl)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 15 degrees C 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 x 10(-13) cm(3)(STP) . cm/cm(2) . s . Pa), lower than most commercial high barrier polymers. At 100% relative humidity, it equilibrated to a T-g of 42 degrees C, well above room temperature. These are polymer systems with high gas barrier properties under both dry and wet conditions.