The bulk properties of polymers are often adjusted via addition of a complex blend of compounds collectively known as additives, where so-called molecular fortifiers (or antiplasticizers) may improve the mechanical properties. In the present work, insight into molecular mechanisms of additive-fortification in model epoxy resins was obtained from multinuclear solid-state NMR analysis. In particular, we have demonstrated a "free molecule"-type behavior of DMSO-d(6) in DMSO-fortified resins similar to common inclusion compounds thereby revealing mere filling of free volume. In case of DMMP-fortified resins, however, chemical modification during postcure of the epoxy resin is observed yielding methyl methylphosphonate (MMP) and salt formation, where dynamic heterogeneities of MMP-d(3) suggest a rather complex mechanism of fortification. The interpretation of NMR data was further supported by oh initio calculations.