A high pressure differential scanning calorimeter (DSC) and a Fourier transform infrared (FTIR) spectrometer with a prism liquid cell (PLC) were used to monitor the reaction kinetics of styrene‐unsaturated polyester resins at elevated curing temperatures and pressures. The thermal method is easy to perform but provides only an overall reaction exotherm. The spectroscopic method can detect the detailed reaction mechanism of copolymerizations. It is, however, less quantitative and the calculation is much more time‐consuming compared to the thermal analysis. Reactions of two unsaturated polyester resins with different molecular structure were measured by these two methods. Results showed that applying cure pressure on unsaturated polyester resins reduced the reaction rate but increased the final conversion. The styrene reaction was enhanced more than the polyester reaction at high curing temperatures.