The antiplasticization of epoxy networks based on diglycidylether of bisphenol A (DGEBA) and hexamethylene diamine was investigated by both dynamic mechanical analysis and high-resolution solid-state C-13 nuclear magnetic resonance (n.m.r.) spectroscopy. Particular attention was paid to the influence of the antiplasticizer on the beta secondary transition. In the case of densely crosslinked networks, the relevant loss peak of the antiplasticized material is dramatically reduced compared with that of the pure resin. The cooperative modes responsible for the high-temperature part of the beta relaxation in the pure networks do not show up in the antiplasticized systems. N.m.r. measurements clearly demonstrate that the loss peak reduction is accompanied by a strong decrease in the mobility of the CH2 groups that are close to the nitrogen crosslinks. In agreement with these observations, the efficiency of the antiplasticizer is weaker in the loosely crosslinked networks where the cooperative modes of the beta transition are reduced. In the quasi-linear systems, where the beta motions tend to be restricted to isolated motions, the efficiency of the antiplasticizer almost vanishes. This molecular description of the antiplasticization mechanism, in terms of hindrance of the short-scale cooperative motions in the glassy state, can be viewed as a dynamic coupling between the polymer and antiplasticizer molecule.