The effects of thickening agent, magnesium oxide (MgO), on the curing behaviour of unsaturated polyester (UP) resins at 110 degrees C were investigated by an approach of integrated reaction kinetics, theology and morphology measurements using d.s.c., Fourier transform i.r. (FTi.r.) spectroscopy, a Haake viscometer, and SEM. For the UP resins thickened with 4% MgO at room temperature for 192 h, isothermal d.s.c. rate profiles at 110 degrees C showed that the induction time, t(z), and the time to reach the maximum rate, t(m), were prolonged and the final overall C=C conversion was reduced when compared with those for the neat UP resins. For the thickened UP resins, FTi.r. measurements revealed that the conversion of styrene was always higher than that of polyester C=C bonds throughout the reaction. Rheokinetic data showed higher gelation time and gel conversion of total C=C bonds than for the neat UP resins. Also, SEM micrographs showed that the fractured surface after the cure appeared less smooth. Finally, all the experimental results have been elucidated in light of the formation of thickening-induced microdomains of aggregates for polyester chain segments prior to cure, as observed by SEM micrographs, and the associated reduction of reactivity for styrene and, especially, polyester C=C bonds in these microdomains during the cure.