Differential scanning calorimetry (DSC), temperature-modulated DSC (TMDSC) and dielectric relaxation spectroscopy have been performed during isothermal curing of an epoxy network (diglycidylether of bisphenol A cross-linked with diaminodiphenyl methane) and of thermoplast modified epoxy resins ("semi-interpenetrating polymer networks") consisting of the epoxy network component and different amounts (10 and 20 wt%) of a linear high T-g-polymer (polyethersulfone). During reaction the homogeneous mixtures phase separate into an epoxy-rich and a linear polymer-rich phase. The time dependent changes in the complex dielectric permittivity are described by a simple two phase model based on two Havriliak-Negami functions combined with Vogel-Fulcher equations for the description of the curing time dependence of the relaxation times. The time increase of the relaxation times in the two phases during isothermal curing is incorporated by time-dependent Vogel-temperatures. The latter are related to the time evolution of the glass transition temperatures T-g in the two phases measured independently by calorimetry. With TMDSC it was possible to resolve T-g for both phases since the curing time evolution of the heat capacity could be separated from the heat of reaction. In the conventional DSC the T-g Of the linear polymer rich phase was masked by the heat of reaction. In addition, with TMDSC the vitrification during isothermal curing could be monitored.