Polymethylphenylsilsesquioxane (PMPSQ-OH) and trimethylsilyl end-blocked PMPSQ (PMPSQ-EC) were prepared. The thermal decomposition behavior of these polymers was studied by thermogravimetric analysis (TGA) and FT-Raman spectroscopy. Hydroxyl-functionalized polystyrene (PS-OH) was also prepared by anionic living polymerization. Thin hybrid films of PMPSQ/PS-OH with various blend ratios were obtained by spin-coating on freshly cleaned glass. The surface morphology of the hybrid films was investigated by atomic force microscopy (AFM). In 80/20 PMPSQ/PS-OH hybrid film, the PS-OH component produced a very uniformly dispersed phase. This hybrid film contained small domains of PS-OH whose size ranged from 60 to 80 nm. As the content of PS-OH was increased, the domain morphology coarsened and phase inversion took place around 50 wt %. In the phase-inversed system, the PMPSQ-rich phase was uniformly distributed in the PS-OH-rich continuous phase. In addition, temperature-dependent dielectric properties of PMPSQ/PS-OH hybrids were investigated. Relaxation of the hybrids was observed with an increasing content of the PS-OH component due to the amorphous glass transition behavior of PS-OH. (C) 2003 Wiley Periodicals, Inc.