Relationships between the microscopic structure (chemical composition and topology) and the macroscopic properties, especially the toughness, of well defined polymeric networks below T(g), were investigated. Model networks were prepared from a triisocyanate, i.e. tris(4-isocyanatophenyl)-thiophosphate (Desmodur RFE) and well defined polymeric diols, namely poly(propylene glycol)s (PPGs) of various molecular weights (425, 725, 1000, 2000, and 4000). The experimental equilibrium rubber moduli of the various Desmodur-PPG networks are in good agreement with those calculated by means of the theory of branching processes. These networks are therefore considered to behave as model polymeric networks. Toughness was measured as a function of the crosslink density at temperatures well below the T(g). The model networks are surprisingly tough. A linear dependence of the critical stress intensity factor, K(Ic), on the strand density is observed. The size of the plastic zone is related to the maximum extensibility of the network chain. Other properties of the glassy state (e.g. glass modulus and yield stress) have only a very slight dependence on the crosslink density.