Minor components present in feed gas streams can have a significant influence on the separation performance of polymeric membranes. Hydrogen sulfide is present in many of the processes where CO2 capture is possible and can therefore undergo competitive sorption with CO2 for transport through the membrane, as well as influence the membrane morphology inducing plasticization. This study investigates the change in CO2 permeability and CO2/N2 selectivity of two glassy polymeric membranes; polysulfone and 6FDA-TMPDA, when 500 ppm H2S is present in the gas mixture. The outcomes of this study reveal that H2S in trace amounts has a strong influence on the separation performance of both membranes. For both membranes, a plasticization partial pressure similar to 0.5-0.6 kPa H2S is observed. H2S competitive sorption is also observed and is modeled by competitive dual-sorption theory. Results suggest that mixed gas permeation influences the amount of each gas immobilized within the Langmuir voids in addition to the expected competitive sorption effects. (c) 2011 American Institute of Chemical Engineers AIChE J, 2012