The kinetic model proposed by Chen and Wu (Chen, S.A.; Wu, K.C. Kinetics of polyesterification. II. Foreign acid-catalyzed dibasic acid and glycol systems. J. Polym. Sci. 1982, 20, 1819-1831.) for the formation of unsaturated polyester resins by reaction between diols and anhydrides has been modified to take into account the difference in reactivity between the two hydroxyl groups of 1,2-propylene glycol and the changes in the concentrations caused by glycol and water removal through distillation during batch polymerisation at elevated temperatures. The specific aim has been to estimate the relative difference in reactivity between maleic and phthalic anhydride to obtain the necessary data for use in computer modelling and simulation studies where the objective was the prediction of coreactant sequence lengths in prepolymers formed by the concurrent reaction of a diol with a mixture of anhydrides. It has been found that the relative reactivity of maleic and phthalic anhydride toward 1,2-propylene glycol after the ring opening of both anhydrides is complete was increased from approximately 1.7 to 2.3 as the temperature is increased from 160 to 220degreesC. There appears to be an anti-synergistic effect in this copolyesterification, i.e., the total rate of reaction of carboxyl groups is lower than the sum of the rates of reaction of the individual acids with the same concentration and at the same temperature. Simulated results are in good agreement with experimental findings.