Quenched systems corresponding to different reaction extents of polyurethane (PU) network formation have been studied using broadband dielectric spectroscopy in order to apprehend the evolution of alpha-relaxation during gel formation. Three PU systems based on poly(oxypropylene) triol of molar mass 260, 700 and 6000 g/mol. have been investigated. While for the pure triols the distribution and temperature dependence of relaxation times are found to be similar, their evolution with connectivity in PU systems depends on molar mass of the triol used. This has been interpreted in terms of relative influence and transformation of active dipoles as a consequence of reaction and evidenced by the appearance of a new dynamic associated to new active dipoles. (C) 2000 Elsevier Science Ltd. All rights reserved.