In this work, the effects of two different diamine monomers containing phosphine oxide on the thermal, mechanical and morphological properties of copolyimides and their hybrid materials were investigated. The gas separation properties of the synthesized copolyimides were also analyzed. The two different diamine monomers containing phosphine oxide were bis(3-aminophenyl]phenylphosphine oxide (BAPPO) and bis[4-(3-aminophenoxy)phenyl)phenylphosphine oxide (m-BAPPO). In the synthesis of the copolyimides, 3,3'-diaminodiphenyl sulfone (3,3'-DDS) was also used as the diamine, as well as 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA). Copolyimide films were prepared by thermal imidization. Furthermore, hybrid materials containing 5 % SiO2 were synthesized by the sol-gel technique. Fourier-transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR) confirmed the expected structures. Dynamic mechanical analysis (DMA) demonstrated that the m-BAPPO-based copolyimides had lower glass transition temperatures (T-g) than the BAPPO-based ones. The thermal decomposition temperature of the m-BAPPO-containing copolyimide without silica was shifted to a higher value. The moduli and strength values of the BAPPO diamine-containing copolyimide and its hybrid were higher than those of the m-BAPPO-containing materials. Contact angle measurements showed their hydrophobicity. Scanning electron microscope (SEM) analysis showed the dispersion of the silica particles in the copolyimides. These copolyimides may be used in the coating industry. The CO2 permeability and the perm-selectivity were the highest, among the other values found in this study, when the m-BAPPO-containing copolyimide in the absence of silica was used. The gas permeabilities obtained from this work were in the decreasing order: P-CO2 > > P-O2 > P-N2.