Previous studies by Benkreira et al.(1) have shown that the critical stage in the distribution of additives in single screw extruders is the zone where the polymer melts. When processing masterbatches and thermoplastics in single extrusion/injection moulding machines, it is suggested that ideally the masterbatch should melt before the host polymer and coat each granule, while remaining sufficiently viscous to promote shear heating and melting of the polymer. This study tests this hypothesis and quantifies the requirements in terms of relative melting points and melt viscosities using image analysis and ’expert’ judgement to rate the mixing performance of the masterbatches.