Polyimide/silica (PI/SiO2) nanocomposite films with 10 wt % of silica content were prepared by sol-gel process under the conditions with and without additional water. The presence of additional water has great effect on the silica particle size and thus on the properties of the prepared PI/SiO2 films. The results indicated that with additional water, the silica particles formed before the imidization of poly(amic acid) (PAA) and aggregated with the increasing of temperature and degree of the proceeding imidization process. For the nonaqueous process, the hydrolysis condensation reaction of tetraethoxysilane (TEOS) did not occur until the imidization of PAA took place, and no silica particles were found in the unimidized PAA films. The hydrolysis-condensation reaction of TEOS was initiated simultaneously by the trace water released from the imidization reaction, the self-catalysis mechanism of the approach provide a means of achieving uniformly dispersed silica particles formed in the PI matrix with particle size in the range of 30-70 nm. (c) 2007 Wiley Periodicals, Inc.