The effect of the molecular weight and acrylonitrile (AN) content of the styrene-acylonitrile copolymer (SAN) on the morphology, mechanical, and rheological properties of SAN/chlorinated polyethylene (CPE) blends was studied. The interaction between dispersed particle and matrix is expected to be optimal at the 25% AN content of SAN. Phase inversion from a dispersion to a continuous phase appears to occur above 50 wt % CPE. Mechanical properties increased with molecular weight at a constant AN content of SAN. Morphological, mechanical, and rheological properties were more sensitive to the AN content, rather than the molecular weight of SAN.