As shown by a precise analysis of the dynamic mechanical data in terms of activation energies and entropies, the extent of the beta transition strongly depends on the role played by the crosslinks. In a densely crosslinked network, localized motions at the spatial scale of an epoxy-amine repeat unit occur at low temperatures, whereas cooperative modes implying more than six units are responsible for the high temperature part of the relaxation. In the quasi-linear systems, only modes with a low spatial extent exist. Measurements of the strength of the C-13-H-1 dipolar couplings, together with other n.m.r. techniques, have shown that the motions of the hydroxypropylether units take part in the beta transition. The DGEBA ring flips also take part in the beta transition. The comparison of dynamic mechanical and C-13 n.m.r. results tends to indicate that the sensitivity of the n.m.r. experiments allows it to probe the cooperative motions, which induce a reorientation of quite a large amount of C-13-H-1 bonds, rather than the isolated ones, whose effect on the C-13-H-1 dipolar coupling is rather weak. In addition, it is shown that the beta transition is broader and more intense in networks prepared in the presence of secondary diamines than in systems with the same crosslink density obtained by using primary monoamines. This result is interpreted in terms of motions that have a higher intramolecular cooperativity than the cooperativity developed in networks incorporating pending hexamethylene units, and a lower spatial extent than the one observed in the densely crosslinked network.