Duroplastic low-stress molding materials, based on epoxy novolak resins with different siloxane modifications have been investigated with respect to the mobility of different structural elements and the morphology of the resulting polymers. Different solid-state NMR relaxation experiments have been used to probe the mobility in a wide range of motional frequencies. Spin diffusion experiments allowed the estimation of domain sizes even in the case of highly filled polymers and indicate that the effect of low-stress modification is correlated with microphase separation.