Recent investigations on Cryogenic Mechanical Milling (CMM) of thermoplastic polymers indicated that macromolecules undergo chain scission leading to the formation of free macro-radicals, crosslinking or amorphization. This makes CMM a promising strategy for the design of highly dispersed blends of thermodynamically immiscible polymers in order to obtain new materials with desirable properties. In this study, CMM is applied on immiscible syndiotactic polystyrene (sPS)/isotactic polypropylene (iPP) blends and compared to conventional melt blending. The resultant blends are characterized by their microstructure in particular their thermal properties. The domain size of iPP in the CMM blends was found to be markedly reduced compared to conventional melt blended samples, although the samples were recrystallized from the molten state. (C) 2004 Kluwer Academic Publishers.