The morphology of isotactic polystyrene (iPS) particles which had been compounded into an isotactic polypropylene (iPP) matrix was varied in-situ by selective heat treatment to be either amorphous (iPS(amorphous)) or semicrystalline (iPS(crystanine)). The influence of iPS morphology on the quiescent and shear-induced isothermal crystallisation of iPP was then studied using differential scanning calorimetry (DSC), in-situ simultaneous small and wide angle X-ray scattering (SAXS/WAXS) and polarised optical microscopy (POM). In the quiescent condition iPS(crystalline) was found to selectively nucleate the beta phase while iPS(amorphous) nucleated the alpha phase of iPP Compared to the control sample (iPP(control)), the presence of both morphologies of iPS increased the number of shear-induced oriented crystal precursors that form as a result of the step shear. During isothermal crystallisation the shear-induced nuclei promote oriented a form crystal growth, accelerate the crystallisation kinetics and ultimately swamp the effect of the nucleating particles present. (C) 2010 Elsevier Ltd. All rights reserved.