To get a better understanding of the curing process of multi-component thermosets differential scanning calorimetric (DSC) and temperature modulated DSC (TMDSC) measurements were performed during isothermal curing of semi-interpenetrating polymer networks (semi-IPNs) with amounts of 10 or 20 wt.% of linear polymer and of the corresponding pure networks at temperatures between 333 and 393 K. The network component consists of diglycidylether of bisphenol A (DGEBA) cross-linked with diaminodiphenyl methane (DDM) and the linear polymer component is polysulfone (PSn) or polyethersulfone (PES). The curing time dependence of conversion was estimated from time dependent heat flow measurements during isothermal curing. The curing kinetics is discussed in the framework of different models taking into account the catalytic effects and the influence of diffusion. A lower reaction rate was found in the semi-IPNs compared with the pure networks which is related to a decrease of the diffusion coefficient and/or the density of reacting units due to the linear polymer component. The final conversions were found to decrease with an increasing amount of the linear polymer component and with decreasing curing temperature which corresponds to less perfect network structures. The time evolution of the glass transition temperatures during isothermal curing was determined by the DSC and TMDSC. At the beginning of the reaction only one glass transition-indicating a one phase system-was found whereas at later stages of curing the two phase structure-consisting of a DGEBA/DDM-rich and a PSn-or PES rich phase-was indicated by two glass transition temperatures.