A homologous series of 12 all-aromatic PEI membranes was investigated with the aim to understand how subtle changes in the PEI main-chain affect the carbon dioxide/methane (CO₂/CH₄) gas separation performance. The 3-ring diamines selected for this study are either para-, meta- or ortho-aryloxy substituted with respect to the central benzene ring, i.e. 1,4-bis(4-aminophenoxy)benzene (P1), 1,3-bis(4-aminophenoxy)benzene (M1) and 1,2-bis(4-aminophenoxy)benzene (O1). Doing so changes the backbone geometry from a more linear to a more kinked conformation. In addition, four dianhydrides were selected with the aim to tailor the segmental mobility and hence the free volume of the PEIs, i.e. pyromellitic dianhydride (PMDA), 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) and 3,3′,4,4′-oxydiphthalic dianhydride (ODPA). We have investigated how subtle changes in these prototypical PEIs affect membrane critical performance criteria such as CO₂ permeability, CO₂/CH₄ selectivity and ability to withstand high operating pressures. In ODPA-based membranes the CO₂ permeability decreases in the order P1 > O1 > M1 and remains steady throughout measurements with mixed feed pressures up to 40 bar, however, the selectivity decreases for ODPA-O1 and ODPA-M1. For high-pressure applications, the OPDA-P1 membrane is a good candidate with a selectivity of 48, permeability of CO₂ of 0.74 Barrer and ability to resist plasticization up to 40 bar of total pressure (16 bar of CO₂ partial pressure). Alternatively, for applications up to 10 bar of total mixed feed (5 bar of CO₂ partial pressure), BPDA-O1 is a promising candidate because this membrane displays a high selectivity of 70 and permeability of 1.3 Barrer.