The permeabilities and diffusivities of CO2 and CH4 at 35 degrees C and atmospheric pressure are reported for two series of modified poly(dimethylsiloxane) membranes, one containing 29.2 to 60 mol% of a monoester and the other 10 to 40 mol% of a diester side-chain functionality. The ratio of the CO2 and CH4 permeabilities, or permselectivity, increases with increasing ester functionality. This enhanced permselectivity results from an increase in the solubility of CO2 relative to CH4 in these membranes, since the diffusivity of CO2 relative to CH4 remains virtually constant as the ester content is increased. Although the number of ester groups in the diester functionalised membranes is twice that of the monoester functionalised membranes at an equivalent level of side-chain substitution, this does not result in an exact doubling of the permselectivity. Mixtures of CO2 + CH4 permeate at atmospheric pressure through diester membranes with unchanged single gas permeabilities irrespective of the composition of the mixtures. However, measurements with single gases at pressures up to 30 bar demonstrate an increasing CO2 permeability caused by plasticisation of the diester membrane, which increases with the degree of functionalisation. The permeability of CH4 is relatively unchanged at elevated pressure.