Acrylonitrile polymer having excellent whiteness and resistance to thermal coloration is prepared by polymerizing acrylonitrile or a mixture of at least 85 percent by weight of acrylonitrile and the balance of an ethylenically unsaturated monomer capable of copolymerizing with acrylonitrile in an aqueous medium containing a catalyst comprising nitrous acid or its salt and a water-soluble sulfoxy reducing compound and at least 10.sup.2 p.p.m. of a chelating agent.
