The processes of gas sorption and permeation in a polymer membrane at temperatures above and below the glass-transition point were examined using poly-4-methylpentene-1 (glass-transition temperature reported to be 40-degrees-C) as a membrane material. The permeabilities to 02 and N2 were independent of applied gas pressure at every temperature; the mean permeability coefficient to CO2 increased with increasing gas pressure. The logarithm of the mean permeability coefficient to CO2 increased linearly with gas pressure due to the plasticization effect induced by sorbed CO2. From the sorption isotherms for CO2 at 20 and 30-degrees-C it was judged that the glass transition was brought about by sorbed CO2 at temperatures below the glass-transition point of the pure polymer.