A series of a-methylstyrene, styrene, and acrylonitrile ((alpha-MSAN) copolymers with different acrylonitrile (AN) contents were synthesized by altering alpha-MSt, St, and AN ratios with emulsion copolymerization method. By melt-blending these copolymers with PVC resin and di-isooctyl phthalate (DOP), PVC/alpha-MSAN, and PVC/(alpha-MSAN/DOP blends were prepared. The miscibility and morphology of the blends were investigated by dynamic mechanical analysis (DMA) and scanning electron microscopy. The PVC is immiscible with SAN by melt-mixing, whereas PVC is miscible with alpha-MSAN (alpha-MSt/St = 1/1) if AN weight percent is within the window range of 2025 wt %, and alpha-MSAN (not containing St) with 35 wt % AN is miscible with PVC even when they are blended by melt-mixing. Replacement of styrene with alpha-methylstyrene widens the miscibility window with PVC. The miscibility of PVC/alpha-MSAN blends is substantially improved with the increasing alpha-MSt content in alpha-MSAN copolymer containing identical AN content. When DOP was introduced into the PVC/alpha-MSAN alpha-MSt/St = 1/1) blends, a single tan delta peak over room temperature in DMA detection is found as AN content in alpha-MSAN copolymer is within the range of 15-25 wt %, and SEM observation also shows that the blends are homogeneous. When the AN content in alpha-MSAN copolymer is over 35 wt %, the presence of DOP causes the phase domain extended. The phase domain size of the PVC/alpha-MSAN/DOP blends intensively depends on AN content in alpha-MSAN copolymer. (C) 2008 Wiley Periodicals, Inc.