When 0.65 mol% toluene was added to SAPO-34 membrane feeds (172-kPa feed pressure), the CO2 and N-2 permeances decreased by approximately 50%, even though toluene is too large to diffuse into the SAPO-34 pores. Single-component adsorption isotherms for CO2 and toluene were used to fit a dual-site Langmuir model to estimate external surface coverages. The lower CO2 permeances qualitatively correlated with the reduced CO2 surface coverages. Changes in permeances for 10-kPa feed pressures were also measured so that surface coverages could be estimated more accurately using ideal adsorbed solution theory. The estimated decreases in CO2 coverages on the membrane surface due to competitively-adsorbed toluene reduced the trans-membrane CO2 driving force and accounted for the observed decreases in permeance.