Model networks have been prepared, at different concentrations in an inert solvent, in such a way that the number of crosslinks is accurately known, and the topology of the network can be varied in a controlled way. The contribution of various network defects to the modulus of the network has been examined. A theoretical estimate of their effect has been made and compared to the experimental result. Two tentative conclusions have been made; first, that the front factor A in the equation for the free energy of deformation of the network has a value of ½ and not the alternative value 1; and secondly that the contribution of entanglements to the modulus of the network varies as the concentration squared rather than as the concentration to the first power.