Synergism on fire properties has been investigated between nano-sized hydrophobic oxides (alumina and silica) and ammonium polyphosphate (AP) flame-retardant additive. Thermal degradation of mixed additives (50% w/w) showed the impact of oxides on AP degradation. The effect of modified nanoparticles was compared with corresponding hydrophilic oxide as regards thermal and fire behaviour. The nanocomposites prepared by melt-blending were evaluated by thermogravimetric analysis and cone calorimetry measurements. Residues were characterized by ATR-FTIR spectroscopy, X-ray diffraction and SEM-EDX experiments. A noteworthy decrease of peak of heat release rate and smoke opacity as well as an increase of LOI were noticed with hydrophobic silica combined with AP both in PMMA and PS. This flame-retardant behaviour was ascribed mainly to the formation of a specific silicon metaphosphate (SiP2O7) crystalline phase which contributes to promote charring and an efficient insulating layer.