Vitrification of a reacting thermosetting system occurs when its glass transition temperature, T-g, rises to the reaction temperature, T. This phenomenon is not restricted to isothermal conditions only : for highly reactive systems, or when the applied heating rate is sufficiently small, vitrification occurs in non-isothermal conditions too. The reaction proceeds in mobility-restricted conditions. Devitrification is observed when the reaction temperature again surpasses T-g of the vitrified resin. The non-isothermal vitrification and devitrification of two epoxy thermosetting systems have been studied using modulated differential scanning calorimetry (MDSC). A normalized mobility factor, which is directly based on the experimental heat capacity evolution, is proposed. For both organic resins, it is shown that the points for which this mobility factor equals 0.5 can be used to quantify the temperatures of vitrification and devitrification. The mobility factor derived from heat capacity is compared to a normalized diffusion factor calculated from heat flow using chemical kinetics modelling. For the epoxy resins studied, both factors coincide. Therefore, the mobility factor can be used as a direct measurement of the change in the rate of reaction when T-g of the reacting system approaches T. Isothermal and non-isothermal MDSC experiments enable the reaction mechanism, the vitrification and devitrification process, and models for diffusion control to be studied, and improved processing conditions for the cure of thermosetting resins to be developed.