A series of model unsaturated polyesters derived from maleic anhydride and various diols were synthesized and a study of their structures was carried out by means of C-13 n.m.r. spectroscopy. The n.m.r. spectra of polyesters produced by different procedures are reported and interpreted in terms of variety in the arrangement of diol components. Both fumarate and ester carbon atoms were found to be sensitive to compositional differences in the polyester chains. Diversity of the diol-fumarate-diol sequences was evidenced to account for the observed splitting of the resonances. Assignment of the peaks to different compositional sequences was accomplished through systematic alterations in polyester composition. The contributions of individual sequences were calculated assuming random and block distribution of diol units and compared with the experimental results. The comparison confirmed the validity of the assignment as well as the efficiency of the approach in the description of diol unit distribution in unsaturated polyesters.