Modulated differential scanning calorimetry (MDSC) is used to study simultaneously the evolution of heat flow and heat capacity for the isothermal cure of thermosetting systems. A stepwise decrease in the heat capacity is observed. For the organic resins studied, it is shown that the glass transition temperature of the curing resin reaches the cure temperature at half of the decrease in heat capacity, Vitrification, the isothermal transition from a rubbery or liquid state to a glassy state, and the corresponding vitrification time can thus be measured in a single experiment. A mobility factor is proposed which is based directly on the observed heat capacity evolution. This mobility factor is compared to the diffusion factor which is calculated from the modelled rate of reaction and the decrease in the rate of reaction at vitrification. For the amine-cured and the anhydride-cured epoxies studied, both factors coincide and thus the mobility factor can be proposed as a direct measurement of the diffusion factor. In contrast, the rate of reaction of the low-temperature formation of an inorganic polymer glass (IPG) is (nearly) uninfluenced by the vitrification process. The conversion increases long after vitrification and a glass transition temperature largely above the cure temperature can be reached, In this case, the diffusion and mobility factors do not coincide. MDSC is shown to be a promising technique to study in more detail the effect of polymer microstructure and reaction mechanism on the interrelation between vitrification and the decrease in the rate of reaction due to mobility restrictions.