The present invention relates to a process for the polymerization of acrylonitrile or a monomer mixture containing acrylonitrile as a main component and at least one other ethylenically unsaturated compound, characterized in that the polymerization is conducted at a temperature above 120.degree. C. under a pressure above the vapor pressure generated in the polymerization system under the polymerization conditions in a system in which water is present in a range of 3 to 50 percent by weight based on the total weight of the monomer(s) and water to produce an acrylonitrile polymer in a substantially molten state. The polymers produced according to the above process have the advantage of being able to be directly shaped by extrusion without the need of solvents.