Both viscoelastic behavior and plastic deformation in compression were investigated on new polythiourethane networks based on diisocyanates and tri- or tetra-thiols. Dynamic mechanical analysis was used to characterize the mechanically-active alpha and beta relaxations as a function of crosslink density and nature of the diisocyanate. Data molecular analysis on these novel materials led to conclusions in good agreement with earlier statements on the well-known epoxy-amine networks. Besides, consideration of the relaxational behavior, and especially in the beta relaxation region, allowed one to interpret the plastic deformation properties of the systems under study and to predict their poor resistance to fracture. By the way, analysis of the energy required to yield permitted determination of a new characteristic temperature, T-y, which can be compared to the thermomechanical main transition temperature, T-alpha, and to the glass transition temperature, T-g. (C) 2007 Wiley Periodicals, Inc.