The miscibility of poly(vinyl chloride)/terpoly(alpha-methylstyrene-acrylonitrile-methyl methacrylate) (PVC/MS-AN-MMA) blend was investigated by differential scanning calorimetry. The parameters of thermal transitions and phase behaviour of the blend were determined. Using the criterion that miscible blend exhibits one glass transition temperature T(g), the miscibility of the blends with 0-40 and with more than 80 mass% of MS-AN-MMA was found. The dependence of glass transition temperature on the composition was well described by the Couchman, Gordon-Taylor and Fox equations. The blends with 50-80 mass% of MS-AN-MMA were partially miscible. The apparent mass fraction of PVC and MS-AN-MMA in the PVC-rich and MS-AN-MMA-rich phases was estimated. The phase diagram of PVC/MS-AN-MMA blend indicated miscibility upon heating.