Unsaturated polyesters and dicarboxy diesters from maleic anhydride and various diols were investigated by means of H-1 and C-13 nuclear magnetic resonance spectroscopy. The carbons of diol units were found to be sensitive to maleate/fumarate transformation, enabling the process to be followed in detail. The cis-trans isomerism of dicarboxy diesters was described and related to the structure of the corresponding diols. The effects induced by the isomerism were examined separately for various carbons and, in the case of 1,2-propylene glycol, direct evidence was provided of the preference of CH over CH2 groups in promoting the trans isomer. The results of the study on dicarboxy diesters along with data concerning the distribution of diol units along the chain were utilized to interpret the isomerism of unsaturated mixed polyesters.