The phase behaviour of blends of four poly(chlorinated-ethyl methacrylates) with poly(styrene-co-acrylonitrile) (SAN) and with poly(p-methylstyrene-co-acrylonitrile) (pMSAN) was studied. Poly(1-chloroethyl methacrylate) and poly(2,2-dichloroethyl methacrylate) are miscible with SAN and pMSAN over a certain copolymer composition range, showing miscibility windows. Poly(2,2,2-trichloroethyl methacrylate) (PTCEMA) is miscible with SAN and pMSAN having high acrylonitrile contents, showing miscibility doors. PTCEMA is also miscible with polyacrylonitrile. However, poly(1,2,2,2-tetrachloroethyl methacrylate) is immiscible with any of the SAN and pMSAN samples.