The sorption of compressed carbon dioxide and methane in a series of all-aromatic poly(etherimide) (PEI) thin films is presented. The polymer films are derived from the reactions between an arylether diamine (P1) and four different dianhydrides [3,3',4,4'-oxydiphthalic dianhydride (ODPA), 3,3',4,4' biphenyltetra-carboxylic dianhydride (BPDA), 3,3',4,4'-benzo-phenonetetracarboxylic dianhydride (BTDA), and pyromellitic dianhydride (PMDA)] that have been selected to systematically change the flexibility of the polymer backbone, the segmental mobility, and the nonequilibrium excess free volume (EFV) of the polymer. The EFV, gas sorption capacities, and sorption-and temperature-induced dynamic changes in film thickness and refractive index have been investigated by spectroscopic ellipsometry. The sorption capacity depends to a great extent on the PEI backbone composition. PMDA-P1 shows the highest carbon dioxide sorption, combined with the lowest sorption selectivity because of the predominant sorption of methane in the EFV. For ODPA-P1, the highest sorption selectivity is obtained, while it shows little long-term relaxations at carbon dioxide pressures up to 25 bar. (C) 2016 Wiley Periodicals, Inc.