Three copolyimide membranes containing the dimethylsiloxane structure were prepared in our laboratory by the condensation polymerization of alicyclic 2,3,5-tricarboxy cyclopentyl acetic dianhydride (TCDA) with diamino diphenyl ether (DADE) and bis(aminopropyl)polydimethylsiloxane (BAS) with differing chain length. The morphology and the transport properties of gases, especially carbon dioxide and methane, were investigated. The copolyimide membranes showed a glassy state at room temperature and microphase separation. The shift of Tg in the DSC curves of polydimethylsiloxane (PDMS) and TCDA-DADE was observed at low levels of the PDMS content, such as 9-12.6 wt.%. The sorption isotherms as well as the permeability coefficients were discussed in terms of the dual sorption and transport model, assuming that the diffusion coefficient is concentration-dependent. The permeability coefficient of carbon dioxide showed a minimum value at around 3-5 atm of the supplied pressure at 35°C depending on the dimethylsiloxane content. The permeation behavior for a carbon dioxide and methane mixture (50/50), when compared with the pure gases, was quite different. In the range of a high supplied pressure up to 0.6-40 atm, the permeability coefficient of carbon dioxide in the pure gas was consistently higher than that in the mixed gas. On the contrary, the permeability coefficient of methane in the pure gas was lower. These phenomena were considered to be due to the plasticizing effect of the dissolved carbon dioxide molecules in the Langmuir's domain of the membranes.