This study investigates the effects of the incorporation of nanoparticles on the reaction to fire of an epoxy resin containing a conventional intumescent flame retardant. Two types of nanoparticles are considered: OctaMethylOligomericSilsesquioxanes (OMPOSS) and Carbon NanoTubes (CNTs). The combination of an intumescent phosphorus-based flame-retardant (APP) and CNTs provides no enhancement of the reaction to fire of this system. In contrast, using OMPOSS in combination with APP provides a large synergistic effect via an intumescence phenomenon. The study of the thermal degradation of these systems shows that the interactions between these fillers modify the viscosity of the degraded matrix. The trapping of degradation gases is enhanced in the case of APP/OMPOSS, which results in the creation of an intumescent protective layer earlier than with the reference system containing APP alone. Furthermore, the presence of OMPOSS permits the creation of silicophosphates which reinforce the residue. In contrast, the residue of the formulation containing carbon nanotubes is excessively stiff and it cracks during combustion, hindering the proper formation of the protective layer.