Isothermal curing of a thermosetting system was investigated by temperature-modulated differential scanning calorimetry (TMI)SC) at different frequencies. From the periodic component of the heat flow, the amplitude and the phase shift were determined. The amplitude mainly delivers information on the thermal relaxation (vitrification process), whereas the phase shift also includes information of the temperature dependence of the reaction rare and the heat transfer conditions. A procedure is introduced to separate these effects. As a result, we get the phase shift that is only due to the thermal relaxation. This is required to determine the correct complex heat capacity. Additionally, the phase shift delivers information on the reaction kinetics, i.e. the change from chemical-controlled to diffusion-controlled kinetics.