The gas sorption behavior of polyimides (PIs) incorporating Tro¨ger׳s base (TB) units was investigated for five representative small gases (H2, N2, O2, CH4, and CO2) by barometric pressure decay methods at 35 °C. Methanol treatment of the PI-TB membranes increased the sorption, diffusion, and permeation of the small gas molecules because of increased interactions between the polymer and gas molecules as well as an increase of the FFV due to dilation of the polymers. Two different solubility determination methods, pressure-decay based and time-lag based methods, were introduced and compared. The gas solubilities of relatively condensable gases such as CO2 and CH4 showed good agreement between the two solubility measurements. Two different dianhydrides (6FDA and BTDA) in the PI-TBs were also introduced to determine their structural contributions to the gas sorption and transport properties. The two PI-TBs showed similar behavior in Henry׳s mode sorption. However, their Langmuir mode sorption demonstrated distinctive behavior, resulting in different gas transport phenomena. Methanol-treated PI-TB membranes exhibited plasticization resistance for CO2 permeation under high CO2 content feed in CO2/CH4 mixture. Incorporating TB units in the PI backbones achieved comparable microporosity and gas permeability results especially for CO2 separation in highly permeable polymer membranes.