In this study, new monomers having siloxane groups were synthesized as an intermediate for preparation of siloxane modified polyimide polymers. Then with these monomers, the synthesis of uncrosslinked and crosslinked polyimide–siloxane hybrid polymer membranes were achieved. The purposes of the preparation of modified polyimides were to modify the thermal and chemical stability, and mechanical strength of polyimides, and to improve the gas separation properties of polymers. The new diamine monomer having siloxane groups was prepared from 3,5-diaminobenzoic acid (3,5-DABA) and 3-aminopropyltrimethoxysilane (3-APTMS) in N-methyl-2-pyrollidone (NMP) at 180°C. The modified polyimide membranes having different amount of siloxane groups were synthesized from pyromellitic dianhydride (PMDA), 4,4-oxydianiline (ODA), and 3,5-diaminobenzamido-N-propyltrimethoxy silane (DABA/PTMS) in NMP using a two-step thermal imidization process. The synthesis of modified polyimide membranes were characterized by Fourier transform infrared spectroscopy (FTIR). The thermal analysis of the polyimides were carried out by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Water absorption and swelling experiments were also carried out for the investigation of structural properties of polymers. FTIR observations confirmed that the polyimide membranes with new diamine intermediate were successfully obtained. Thermal analysis showed that the uncrosslinked copolyimides exhibited two glass transition temperatures, indicating that they were separated microphases and it was found that all the modified copolyimides had showed higher glass transition temperature (Tg) than unmodified polyimides. The separation properties of the prepared polyimide membranes were also characterized by permeability for O2 and N2 gases and ideal selectivity values were calculated. Copyright © 2009 John Wiley & Sons, Ltd.