Hydrogenated segmented poly[butadiene-block-((styrene-co-acrylonitrile)-block-butadiene)(n)] block copolymers, which were developed by use of the polymeric iniferter technique, were tested for their compatibilizing capacities for (10/90) LDPE/PVC blends. The acrylonitrile content of the SAN blocks of the block copolymers was, as expected, found to be an extremely important factor for their miscibility with the poly(vinyl chloride) phase. When the SAN blocks of the block copolymers have the proper composition, they are excellent agents for the dispersion of the polyethylene phase in the blend into smaller domains. Addition of one percent of block copolymer proved to be sufficient to give a large improvement of the mechanical properties of the blend. These compatibilized blends showed toughnesses comparable to the virgin poly(vinyl chloride). Scanning electron micrographs showed improved adhesion between the dispersed polyethylene phase and the poly(vinyl chloride) matrix.