According to the viscosity change during curing reaction the glass transition temperature increases. At isothermal conditions or slow heating, a thermosetting resin vitrifies during curing reaction if the reaction temperature is lower than the actual glass transition temperature of the reacted material. This is indicated by the heat capacity change during reaction. Due to the vitrification process the kinetics become diffusion-controlled and the reaction rate decreases dramatically. The transition between chemically controlled and diffusion controlled reaction can be described using a diffusion control function. The actual reaction rate can be expressed as a product of the reaction rate of the chemically controlled reaction and the diffusion function. It is shown that the chemically controlled kinetics can be evaluated using iso-conversional methods (model-free kinetics) from DSC heating experiments at sufficiently high heating rates. Several approaches of the diffusion function exist. However, most of them need a large number of parameters or measured data of the vitrification process. Using such approaches the material behavior during diffusion control cannot be predicted, because the parameters have to be measured before. To overcome such drawback we introduce a phenomenological expression for the diffusion control function, which is independent of the reaction temperature and requires the conversion dependence of the glass transition temperature. The presented approach can be used to predict the kinetics of complex reactions, including the change from chemically controlled to diffusion-controlled kinetics and the behavior in the diffusion controlled regime, on the basis of a small number of experiments. The tested system is a thermoset and consists of an epoxy resin (DGEBA) and a cross-linker (DDM). This system is studied by DSC, Temperature Modulated DSC and Dynamic Mechanical Analysis (DMA).