The influence of processing temperature and the presence of the reactive modifier, m-phenylene bismaleimide (BMI), on the formation of highly filled polyethylene/Mg(OH)(2) composites has been investigated using central composite design methods. Infrared spectroscopic and differential scanning calorimetric evidence produced suggests that maleimide C=C bonds are involved in a free radically induced process which leads to extended and crosslinked polyethylene chains being produced, some of which encapsulate the filler particles, resulting in formation of a crosslinked interphase. This effect resulted in increased tensile strength and reduced composite melt flow rate (MFR).