Hydrolysis and condensation proceeded to generate 3-aminopropyl polysilsesquioxane-like (APPSSQ-like) particles from 3-aminopropyl triethoxysilane. An APPSSQ-like particle includes two domains: crosslinking Si-O-Si networks inside and 3-aminopropyl groups outside the particle. The APPSSQ-Iike/polyimide (APPSSQ-like/PI) hybrid composites were prepared from a solution of poly(amic acid) (PAA, polyimide precursor) and APPSSQ-like particles using N,N-dimethylacetamide as a solvent. Scanning electron microscopy (SEM) showed that the APPSSQ particle sizes were about 100 nm, and there was no obvious phase separation between the APPSSQ-Iike particles and the PI matrix in the fracture surfaces of the hybrid films. Owing to the highly crosslinked Si-O-Si networks inside the APPSSQ-Iike particles, the APPSSQ-like/PI hybrid composites possessed desired properties such as improved thermal resistance and reduced coefficients of thermal expansion (CTE). The presence of covalent bonds be-tween the APPSSQ-Iike particles and the PI molecules improved the compatibility between these two components. Thus, the tensile strength increased with the APPSSQ-Iike content, and the elongation at break also slightly increased with the APPSSQ-Iike content. The initial tensile and storage moduli of the APPSSQ-like/PI hybrid composites increased with the APPSSQ-Iike content, which indicates that the mechanical properties of these hybrid composites were enhanced by the incorporation of the APPSSQ-Iike content in the PI matrix. Furthermore, the glass transition temperatures of these composites increased with the APPSSQ-Iike content because the adhesion between the APPSSQ-like particles and the PI molecules restricts the PI chains' mobility. (C) 2003 Wiley Periodicals, Inc.