Minor components present in polymeric membrane gas separation can have a significant influence on the separation performance. Carbon monoxide and nitric oxide exist in post-combustion gas streams and can therefore influence CO2 transport through membranes designed for that application. Here, the permeability of nitric oxide (NO) through three glassy polymeric membranes (polysulfone, Matrimid 5218 and 6FDA-TMPDA) was determined and found to be less than the CO2 but greater than the N-2 permeability in each membrane. This study also investigated the influence of 1000 ppm CO on the mixed gas permeability of CO2 and N-2 for two glassy polymeric membranes; polysulfone and 6FDA-TMPDA. For both membranes, CO competitive sorption resulted in a reduction in the measured permeability of CO2 and N-2 even though present at only low concentration. (C) 2011 The Institution of Chemical Engineers. Published by Elsevier BM. All rights reserved.