In this study, the chemical modification of unsaturated polyesters and the influence of polyester's structure on thermal and viscoelastic properties have been presented. The structure of unsaturated polyesters obtained in polycondensation of cyclohex-4-ene-1,2-dicarboxylic anhydride (THPA), maleic anhydride and only one suitable symmetrical glycol: ethylene glycol or 1,4-butanediol (BDO) or 1,6-hexanediol has been modified by peracetic acid. The selective oxidation of unsaturated polyesters conducted in mild time and temperature conditions was a successful and effective method to prepare new materials/unsaturated epoxy polyesters/containing epoxy groups in cycloaliphatic rings and carbon-carbon double bonds in polyester chain. The unsaturated epoxy polyesters were capable of both copolymerization with vinyl monomer and polyaddition reactions with suitable curing agent. Therefore, they were successfully used as a component of low styrene content copolymers. As was confirmed by DSC, DMA, and TGA analyses, polyester's structure had significant influence on thermal and viscoelastic properties of styrene copolymers. The properties of styrene copolymers prepared from unsaturated epoxy polyesters were considerably better compared with those obtained for styrene copolymers from unsaturated polyesters. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 720-731, 2009