Dielectric relaxation spectroscopy measurements have been performed during isothermal curing of semi-interpenetrating polymer networks (semi-IPNs) and the related pure networks at frequencies between 3 kHz and 3 MHz and curing temperatures between 313 and 393 K. The pure networks consist of diglycidyl ether of bisphenol A (DGEBA) cross-linked with diaminodiphenylmethane (DDM) and the semi-IPNs contain in addition 10 or 20 wt% of polysulfone (PSn) as the linear component. Temperature dependent dielectric measurements have been performed on the fully cured samples in the temperature range from 130 to 550 K for the same frequencies. For the pure networks, the imaginary part of the complex permittivity ε″ shows a decrease with curing time tcure followed by a peak. For the semi-IPNs this peak is much broader for all cases and can be resolved into two maxima for several curing conditions. The decrease in ε″ s connected to the decrease in the dcconductivity due to gelation, whereas the peak is related to the relaxation of dipoles. The existence of two maxima in the time dependence of ε″ is an indication for two different relaxations in a phase separated structure. This is supported by temperature dependent measurements on the fully cured samples. The characteristic relaxation times T, which are extracted from the maxima of ε″ (tcure) forr different frequencies /using the relation ωτ = 1(Ε = 2πf), increase during curing by several orders of magnitude for both the pure networks and the semi-IPNs. In order to extract the characteristic times for gelation tg and the times t0 where the dc-conductivity σdc approaches a singularity, σdc was fitted by a power law, σdc = σ0[(tg - t)/tg]p, and an exponential function, σdc = A exp [B/(t - t0)], respectively. In the error limits of the experiments both procedures lead to similar results. The tg and t0 values are in good agreement with those measured mechanically and no significant differences between pure networks and semi-IPNs cured at the same temperature were found.