The viscoelastic and dielectric properties of polypropyleneglycol (PPG)-tris-(4-isocyanatophenyl)thiophosphate (Desmodur®) model networks have been studied. Attention was focused on the relaxation behaviour near the glass transition. Two relaxation mechanisms were observed in the glass transition range, one due to the PPG chain segments and one due to the less mobile cross-link moieties, i.e. the networks show motional heterogeneity. Mechanically only the first mechanism was observed while dielectrically both mechanisms were found. This motional heterogeneity has a profound effect on the dependence of the glass transition temperature on the composition of these networks. The effect of the stage of cure on the temperature of maximum loss and the magnitude of these two relaxation mechanisms was studied dielectrically. Finally, the effect of exchanging the cross-linker molecules with chain extender molecules with similar structure, thus lowering the cross-link density, was studied.