Polyimide (PI)/poly(vinylsilsesquioxane) (PVSSQ) (PI/PVSSQ) hybrid composite films were prepared from 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA)-4,4'-oxydianiline (ODA) polyamic acid and triethoxyvinylsilane (TEVS or VSSQ) via sol-gel process and thermal imidization. The presence of the PVSSQ showed a remarkable effect on the microstructure and properties of the polyimide based hybrid films. The transparency of the hybrid films decreased with increasing the content of the PVSSQ. The compatibility and interfacial interaction of the hybrid composites were evaluated by scanning electron microscope (SEM) and atomic force microscope (AFM), respectively. The PI/PVSSQ hybrids showed nanocomposite formation when the contents of PVSSQ was less than 20 wt%. It was found that the surface topography was influenced by the composition of the PVSSQ. Incorporating of the PVSSQ increased the thermal stability and T-g of hybrid composites. The dielectric constant of the hybrid composites was reduced by adding PVSSQ up to a certain content, showed a minimum and then found to be increased. The dielectric constant values of the hybrid composites ranged from 2.59 to 3.78. The presence of the PVSSQ also showed significant effects on the mechanical properties of the polyimide films. (C) 2003 Elsevier Science Ltd. All rights reserved.