Gas sorption and transport properties of poly(tertiary-butyl methacrylate) (PtBMA) have been measured at 35 degrees C and are compared with prior results for poly(methyl acrylate) (PMA), poly(methyl methacrylate) (PMMA), and poly(ethyl methacrylate) (PEMA). The t-butyl substitution increases the gas permeability as a result of increases in both diffusivity and solubility coefficients; however, the ideal selectivity for a given gas pair may increase or decrease depending upon the methacrylate polymer to which it is compared. Poly(t-butyl methacrylate) does not show plasticization by CO2 up to 10 atm of CO2. The effect of t-butyl substitution on gas permeability observed here for the methacrylate polymer is compared to that reported in the literature for other polymer types. This substitution causes a larger increase in permeability the smaller the repeat unit of the base polymer since this results in a greater modification in structure and subsequent greater decrease in efficiency of chain packing.