The kinetics of the individual key reactions involved in the formation of unsaturated polyester resins have been studied using the very reactive glycol, 2-methyl-1,3-propanediol (MPD). This diol has been reacted in turn with maleic anhydride (MA), phthalic anhydride (PA) and isophthalic acid (IA) under isothermal conditions in the temperature range 180-210degreesC and the kinetic constants for the following reactions have been obtained MA + MPD, PA + MPD, IA + MPD, MA + PA + MPD and MA + IA + MPD. The relative reactivities of MPD with MA and PA measured by monitoring the loss of carboxyl groups at 180 and 200degreesC were found to be 2.26 and 1.70, respectively. At 200degreesC PA is more reactive than IA (ratio approximately 1.3 1) in homopolyesterification. In the copolyesterifications involving PA and IA where cross catalysis can occur, the PA reacted approximately 1.25 times faster than IA at 200degreesC. The differences in reactivity might be expected to have a significant effect on the coreactant sequence lengths in prepolymers formed by the concurrent reaction of PA, IA and MPD particularly at low conversions thus on the final properties of the cured resins in which they are employed. The properties of the products are not examined in this paper. (C) 2003 Published by Elsevier Ltd.