The removal of carbon dioxide from natural gas by membrane gas separation is an important process ill upgrading the gas for Consumers. Hydrocarbons associated with natural gas have the potential to influence the CO2 separation performance of membranes. Here, a glassy polymeric membrane of poly (4,4'-hexafluoroisopropylidene diphthalic anhydride-2,3,5,6-tetramethyl-1,4-phenylenediamine) (6FDA-TMPDA) was exposed to hexane and toluene at different partial pressures in a 90% CH4-10% CO2 gas Mixture. The permeability of CO2 and CH4, as well as the selectivity, declined over time as both hexane and toluene competitively sorbed into the membrane. The resulting behavior fit the dual-sorption model for polymeric membrane separation and indicated Langmuir affinity constants of 0.13 +/- 0.03 atm(-1) for methane, 240 +/- 100 atm(-1) for hexane, and 980 +/- 500 atm(-1) for toluene (fugacity basis). The kinetics of the permeability decline were faster than that of the permeability recovery after the hydrocarbon contaminant was removed which may indicate that desorption from the Langmuir voids is the rate controlling step.