Thermoplastic compositions comprise blends of 25 to 65 parts by weight of thermoplastic SAN resin and 75 to 35 parts by weight of a nitrile rubber per 100 total parts by weight of resin and rubber, in which the rubber is present as a dispersion of particles of the rubber in a matrix of the SAN resin. the rubber having a toluene-insoluble gel content of at least 85%. Preferred compositions having both thermoplastic and elastomeric properties comprise a blend of 25 to 40 parts by weight of SAN resin and 75 to 60 parts by weight of nitrile rubber. A preferred process for the production of compositions of the invention comprises continuous mastication of a blend of SAN resin and nitrile rubber with curative for the rubber at curing temperature until the required extent of cross-linking of the rubber is attained.