The characteristic infra-red absorption bands for the cis-and traps-forms of the polyester from malefic anhydride-ethylene glycol have been identified by comparison with the absorption bands of the corresponding model compounds, i.e., diethyl maleate and diethyl fumarate: Cis-form-825, 1410, 3052 cm^-1;traps-form-780, 1371, 3072 cm^-1.The ratio of the optical densities of the 825 and 780 cm^-1 bands has been chosen for the deterurination of the ratio of cis-and trans-forms present in the polyester.Increased temperature and duration of polycsterification increased the traps-content of the polyester obtained.The cis-trans isomerization reaction has been shown to be a first order reaction with an activation energy of 19 kcalimole.The large increase of viscosities of reaction mixture during polyesterification or malefic anhydride and ethylene glycol is mainly due to the increase of trans-content of the polyester formed.