A rate model based on a reaction mechanism was proposed for the melt polyesterification of dicarboxylic acids with diols for the case that no catalyst is added. The model includes the esterification, the cis-trans isomerization and the double bond saturation reactions. All three reactions are acid catalyzed. The overall reaction order of the main reaction increases with conversion due to a shift of the ionic equilibria in the liquid phase. This increase of the reaction order was described with a semi-empirical function. Experiments were carried out for the esterification of maleic acid with propylene glycol at 160-220 degrees C in a laboratory scale batch reactor. The derived rate model successfully described the isomerization of maleic acid to fumaric acid, the saturation of the double bonds, and the polyesterification of maleic and fumaric acids. The model included two parameters for each of the three types of reactions taking place simultaneously.