The effect of temperature on the curing kinetics of two unsaturated polyester resins has been studied by Fourier transform infrared spectroscopy. It has been found that at the beginning of the reaction, the kinetic mechanism was dominated by near-azeotropic copolymerization, while conversion of polyester vinylene groups became much more favourable than styrene later in the reaction. Both styrène and vinylene group conversion at the end of isothermal cures were a function of the styrene content in the resin. The experimental results have been related to the rheological changes during cure. Activation energies have been obtained from both rheological and thermal techniques.