A hydrogenated poly(styrene-co-butadiene) (hSBR) was found to be miscible with isotactic polypropylene (iPP) above the melting point of iPP. The mixture was phase-separated at lower temperatures, i.e. the iPP/hSBR blend exhibited upper critical solution temperature (UCST) type phase behaviour (UCST approximate to 100 degrees C). The UCST phase behaviour was determined by time-resolved light scattering analysis. In the quenched 50/50 blend, a microphase-separated structure of hSBR domains, having a diameter of 20 nm, dispersed quite regularly in an iPP-rich matrix (periodic distance approximate to 40 nm) was observed by transmission electron microscopy. The microphase-separated structure seems to originate from spinodal decomposition below the UCST during the quenching process. The formation of large and ordered lamella crystallites was suppressed to yield fine PP crystallites (of size approximate to 8 nm. as estimated by the Sherrer equation). This may be caused by the presence of the hSBR. Thus, the partially miscible impurity (hSBR) produces fine iPP crystallites which can act as crosslink points to provide thermoplastic elastomer-type character to the blend.