The effect of the Mg(OH)(2) particle size on the thermal degradation and flammability of a model nanocomposite is presented. In order to investigate the effect of particle size on the flammability and thermal stability of materials intended for cable coatings, the ethylene-vinyl acetate copolymer (EVA) was filled with two types of magnesium hydroxide (Mg(OH)(2)) with the average particle size 20 nm and 2000 nm. The thermo-gravimetric experiments performed under nitrogen and air atmosphere did not reveal any substantial effect of particle size on the mechanisms and kinetics of Mg(OH)(2) decomposition. Both types of Mg(OH)(2), at temperatures above 350 degrees C, decomposed endothermally to MgO and H2O. At the same filler volume fraction, composites with the larger Mg(OH)(2) particles exhibited greater value of the limiting oxygen index (LOI) compared to Mg(OH)(2) nanocomposites. It is proposed that the reduction of Mg(OH)(2) particle size below 200 nm results in the catalytic effect of Mg2+ to become more prominent compared to coarser particles. In the case of nano-sized Mg(OH)(2), a large portion of polymer matrix is in direct contact with filler surface due to the high specific surface area of the filler used. Large filler-matrix contact area most probably resulted in the surface-induced catalytic effects of the Mg2+ ion on degradation of polyolefins, already described in the literature, accelerating thermal degradation of EVA matrix. This process acts against the flame retarding effect of the Mg(OH)(2) thermal decomposition. Furthermore, larger filler surface of nano-filler led to the indispensable change of the deformation response of EVA compared to the micro-filled composite. (C) Koninklijke Brill NV, Leiden, 2010