In this paper, the phase separation process of the polyimide/silica hybrid films made from polyamic acid (PAA) and precursor (TEOS-A) hydrolyzed tetraethoxysilane under acidic condition in N-methyl-2-pyrrolidone (NMP) through sol-gel method was investigated by the scanning electron microscope (SEM). A double phase separation was discovered for the preparation of the hybrid films. With evaporation of the solvent NMP at lower than 100 degrees C, the component miscibility of TEOS-A and PAA decreases so that the first phase separation took place and a larger particle phase of TEOS-A precursor with size around 2.0 mu m was formed. The second phase separation from the matrix phase appeared, as PAA was imidized at elevated temperature, which destroyed the interaction between carboxyl group of PAA and hydroxyl group of TEOS-A, and a nanoscale SiO2 particle phase formed. The formation mechanism of the double phase separation was explained by the "capturerelease" model. According to the model, the second phase separation can be controlled by synthesizing amic acid-imide copolymer with different contents of carboxyl group. (C) 2007 Published by Elsevier Ltd.