Polyamide 12 (PA12)-polyisoprene (PIP) particles with a thermoplastic core and a rubbery shell have been prepared in a thermoplastic matrix (PS) by reactive blending. For this purpose, an anhydride-end-capped PS-b-PIP diblock (PS-b-PIP-anh) has been reacted with PA12 chains end-capped (50% of them) by a primary amine and dispersed in PS. A PS-b-PIP-PA12 triblock has been formed at the interface between the PS matrix and the dispersed PA12 microdomains. Thus, the phase morphology consists of PA12 core-PIP shell particles dispersed in PS. The nonreacted PA12 with respect to PS-b-PIP-anh dictates the size of the polyamide (core) domains, and the reactive diblock (mainly the molecular weight of the PIP block) imposes the shell thickness and modulates the core size by its capacity to compatibilize PA12 and the thermoplastic matrix.