The incorporation of small amounts of a chlorinated hydrocarbon wax into polypropylene(PP) when making mica-PP composites is shown to produce a layer of degraded chlorinated hydrocarbon at the interface between the polymer and the reinforcement. Mica is shown to accelerate the thermal degradation of the chlorinated hydrocarbon, and the chlorinated hydrocarbon, or its degradation products, are shown to react with hydrocarbon analogues of the polymer. As well as modifying the chemical nature of the interface, the layer of degraded chlorinated hydrocarbon is shown to modify the morphology of the interface by inhibiting the ability of the mica to nucleate the crystallisation of PP from the melt.