During molecular growth reactions, molecular mobility decreases. Experimentally, an increase in the relaxation time can be detected in the course of isothermal network formation. This process was studied by differential scanning calorimetry (DSC) and temperature modulated DSC using the curing reaction of an epoxy-amine thermosetting system as a classic example. In particular, the conversion dependence of the relaxation time, the heat capacity and the glass transition temperature were investigated. Molecular details of the reaction, i.e. the relaxation and the vitrification are discussed. It is shown, that the change in relaxation behavior during the isothermal cross-linking of chemically unstable material can be described in the same way as the virtual cooling experiment of chemically stable material. Based on this concept, a new relationship for the conversion dependence of the glass transition temperature is derived. It appears that the change of the glass transition temperature as a function of the conversion is directly related to the molecular relaxation processes.