Two unsaturated polyesters (UPs), PME and PMPE, were synthesized by condensation polymerization, and another UP, BPMPE, was prepared from PMPE by blocking of its carboxyl and hydroxyl end groups. Optically transparent UP-silica nanocomposites were prepared by a sol-gel process of alkoxysilanes in the presence of the UPs. Compared to the unblocked UPs, PME and PMPE, the blocked UP, BPMPE, afforded transparent nanocomposites in a wider range of feed ratio of UP/alkoxysi lane. Intermolecular hydrogen bonding and pi-pi interaction between the organic and inorganic components were dominant factors in the nanocomposite formation when acetone was used as a solvent. Various solvents were tested and it was thought that hydrogen bonding acceptor property and boiling point of the solvents were important factors in the formation of the transparent nanocomposites. Photo-crosslinking of an UP in its nanocomposite resulted in the formation of an interpenetrating polymer network structure. (c) 2007 Wiley Periodicals.