A process for the preparation of vinylaromatic (co)polymers grafted on an elastomer comprising a rigid matrix consisting of vinylaromatic polymers or copolymers and of an elastomeric phase dispersed in the matrix in form of particles with a strictly bimodal distribution which comprises the functionalization of a first elastomeric fraction by means of a catalytic system containing a stable free radical initiator and subsequently the polymerization of the thus obtained mixture also in presence of a second non functionalized elastomeric fraction.