Hybrid polyetherimide (PEI)-silica membranes were synthesized. The aim was to obtain improved materials for gas separation media. The inorganic material was prepared via the sol-gel method through the hydrolysis of tetraethoxysilane (TEOS). The influence of the reaction conditions on the final membrane morphology and properties were studied. Scanning electron microscopy (SEM), energy-dispersive X-ray analysis (SEM-EDX), and Fourier transform infrared spectroscopy (FTIR) were used to characterize the PEI and PEI-silica composite membranes. The evolution of TEOS hydrolysis and the condensation processes were verified by FTIR studies. The silica-polymer interaction was also analyzed. The SEM micrographs showed how the membranes distinct morphologies depended upon synthesis parameters and preparation techniques (presence of coupling agent, TEOS polymerization in situ or not, silica content and membranes redissolution). The permeation rates of CO(2), CH(4), O(2), N(2), and H(2) through the pure polymer and hybrid membranes were measured and showed an increase of gas permeability for hybrid membranes but, the CO(2)/CH(4) and O(2)/N(2) selectivities decreased compared to PEI membranes.