In this work, the curing kinetic of an unsaturated polyester resin, mixed with styrene as curing agent, was studied by means of diffential scanning calorimetry (DSC) and infrared (IR) spectroscopy. Investigations were made in situ during curing and post-curing periods. The enthalpy (Delta H-cop) characterizing the styrene homopolymerization and its copolymerization with the polyester chains was determined from isothermal DSC investigations. The residual enthalpy (Delta H-res) was determined from nonisothermal DSC measurements. We find that the quantity (Delta H-res + Delta H-cop) depends on the curing temperature (T-iso). Styrene homopolymerization and/or copolymerization lead to characteristic bonds modifications. The variations of the most characteristic bonds versus curing duration were studied from Fourier transform IR investigations. The data allow the determination of a relationship between transformation rate and curing duration using a time constant (tau) characteristic of each reaction involved during the liquid to solid-state transformation.