Thermoplastic polyetherimide (PEI)-SiO2 nanocomposites were prepared from asoluble PEI, which was synthesized from m-phenylenediamine and bisphenol Adianhydride, in combination with tetraethoxysilane solution via a novel sol-gelprocess. A coupling agent was used to enhance the compatibility between PEI andsilica. This approach was compared with PEI-clay nanocomposite in whichmontmorillonite was modified with ammonium salts of 12-aminododecanoic acidusing an intercalation polymerization. The size and dispersion of the silica or clay inthe PEI nanocomposites were analyzed by x-ray diffractometer and scanning electronmicroscopy. It was found that the sol-gel process offered a fine interconnected or cocontinuousphase, whereas the clay remained dispersed in nanocomposites. Thoughthe thermal properties of PEI-clay nanocomposites were improved over pristine PEI,physical testing showed that the films become brittle as the organoclay contentincreased to over 2%. The thermal stability and mechanical properties of the PEI/silicananocomposites prepared by sol-gel process were improved with silica content up to10%. The onset decomposition temperatures were 550-600°C. The dielectric constantdecreased with increasing amounts of silica. At higher silica contents, the mechanicalproperties were reduced as a result of the phase separation.