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 (01). 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 > 01 > Ml and remains steady throughout measurements with mixed feed pressures up to 40 bar, however, the selectivity decreases for ODPA-01 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.