Magnesium hydroxide is of increasing interest as an environmentally friendly flame retardant filler for ethylene vinyl acetate (EVA) copolymers which are widely used in cable applications. In this paper, magnesium hydroxide (Premier Periclase DP393) was used at a flame retardant loading of 60% w/w in an EVA copolymer with a vinyl acetate content of 18% (molar). The particular modifiers used in this study were stearic acid (converted to ammonium stearate (AS)) and gamma-aminopropyltriethoxysilane (aPS). The effect of surface-treated magnesium hydroxide on the processing, mechanical and the flame retardant properties were determined, together with characterisation of the interaction between filler and polymer. These studies have shown that, relative to the untreated filler, use of AS coating leads to a decrease in tensile strength to a limiting value at monolayer coverage, but an increase in elongation at break, which reaches a maximum at monolayer coverage. However, the APS treatment enhanced both tensile strength and elongation: relative to untreated filler, when used at levels in excess of monolayer. Characterisation studies suggest that this effect was related to the silane treatment promoting increased filler-matrix adhesion, and at levels greater than monolayer, promoting formation of a thick interphase and possible crosslinking of the bull; matrix.