Thermoplastic polyetherimide (PEI)-SiO2 nanocomposites were prepared from a soluble PEI, which was synthesized from m-phenylenediamine and bisphenol A dianhydride, in combination with tetraethoxysilane solution via a novel sol-gel process. A coupling agent was used to enhance the compatibility between PEI and silica. This approach was compared with PEI-clay nanocomposite in which montmorillonite was modified with ammonium salts of 12-aminododecanoic acid using an intercalation polymerization. The size and dispersion of the silica or clay in the PEI nanocomposites were analyzed by X-ray diffractometer and scanning electron microscopy. It was found that the sol-gel process offered a fine interconnected or co-continuous phase, whereas the clay remained dispersed in nanocomposites. Though the thermal properties of PEI-clay nanocomposites were improved over pristine PEI, physical testing showed that the films become brittle as the organoclay content increased to over 2%. The thermal stability and mechanical properties of the PEI/silica nanocomposites prepared by sol-gel process were improved with silica content up to 10%. The onset decomposition temperatures were 550-600 degrees C. The dielectric constant decreased with increasing amounts of silica. At higher silica contents, the mechanical properties were reduced as a result of the phase separation.