In this study, oligomeric phenylsilsesquioxane (OPSQ)-titania optical thin films were synthesized and characterized. OPSQ with the end group of Si-OR was prepared first. Then, it was reacted with titanium (IV) n-butoxide, followed by spin-coating and multi-step curing to form the optical thin films. Highly homogeneous and transparent films were obtained at the titania content of 0-54.8 wt.%. The titania domain in the prepared films was estimated to be 8 nm from the TEM diagram. The optical properties of the prepared hybrid films could be tuned by the titania content. By increasing the titania content from 0 to 54.8 wt.%, the absorption edge and refractive index of the prepared film were increased from 277 to 322 nm and 1.527 to 1.759, respectively. These results could be explained from the growing size effect of the titania domain. The optical loss of the studied planar waveguide by using the OPSQ-titania as the core layer decreased from 0.568 to 0.415 dB cm(-1) as increasing the titania content from 0 to 15.9 wt.%. It is resulted from the reduction of the C-H bonding density of the hybrid materials by increasing the titania composition. The prepared films could have potential applications for high refractive index coating or optical waveguides. (C) 2003 Elsevier B.V. All rights reserved.