The influence of screw speed, agitator configuration, and compatibiliser, namely maleated polypropylene (mPP), during twin-screw extrusion upon the morphology, dispersion stability, and properties of polyamide-6/polypropylene (PA6/iPP) blends has been investigated. For mPP contents of 2.4 vol% and greater, the number average domain diameters measured in the extruded compound, D (ex), were found to decrease to a limiting value at the specific mechanical energy input, S,, around 0.07 kW h kg(-1) when S, was controlled by the screw configuration and screw speed. For mixing energies greater than 0.07 kW h kg(-1), the domain size was principally determined by the ratio of specific interfacial area, S-ext, between the PA and iPP phases to the estimated volume fraction of in situ formed copolymer, phic; S-ext/phi(c) approximate to 45 mum(-1). The difference, DeltaD(n), between D-n(ex) and the average domain diameter measured in the cores of the samples after injection moulding, D-n(m), was calculated: DeltaD(n) = \D-n(ext) - D-n(m)\. It was found that DeltaD(n) proportional to D-n(ext); i.e. larger droplets in the extrudate underwent the greatest changes in dimension in the subsequent moulding process.