Acrylonitrile is polymerized, if desired together with other ethylenically unsaturated compounds containing a terminal methylene group, in an aqueous medium at a pH below 7 using radical-forming activators in the presence of 0.001 to 10% by weight of the monomer of a condensed phosphoric acid which contains at least three phosphorus atoms per molecule and 1 to 250 ppm by weight of the monomer of a compound of a metal of Group Ib, Va, VI(a) and VIII of the Mendel aeef Periodic System, the total quantity of catalysts used being 0.2 to 10% by weight of the monomer. Specified comonomers are methacrylonitrile, acrylic and methacrylic acids and esters, vinyl chloride, vinylidene chloride, vinyl esters, styrene sulphonic acid, b -methacryloyl-amino-ethanol sulphuric acid ester, vinyl pyridine, acryloylaminoethanol and N-methacryloyl-N1-dimethyl ethylenediamine. Specified condensed phosphoric acids are metaphosphoric and isometaphosphoric acids and acids prepared by dehydrating orthophosphoric acid. Specified metal compounds are compounds of iron, vanadium and copper, particularly ferrous sulphate and copper sulphate. Specified radical-forming activators are hydrogen peroxide, persulphates, perphosphates, perborates, azo compounds and mixtures of chlorate and bisulphite, persulphate and pyrosulphite, perborate and bisulphite, perphosphate and pyrosulphite, and persulphate and sulphur dioxide. The reaction mixture may be acidified with acids such as sulphuric, nitric, perchloric, orthophosphoric, amidosulphonic, toluenesulphonic and chlorinated acetic acids. Examples describe the copolymerization of acrylonitrile with (1) methyl acrylate, (2 to 6 and 8 to 10) methyl acrylate and methacroyl aminobenzene benzenedisulphonimide, and (7) methyl acrylate and styrene sulphonic acid using catalyst systems comprising (1, 2 and 4 to 7) potassium persulphate, sodium pyrosulphite and iron, (3) ammonium persulphate sodium pyrosulphite and iron, and (8 to 10) potassium persulphate, sodium pyrosulphite and copper, in the presence of (1, 4 and 7) metaphosphoric acid, (2 and 4 to 6) metaphosphoric acid and sulphuric acid, (3) metaphosphoric acid and chloracetic acid, (8) metaphosphoric acid and toluene-sulphonic acid, (9) dehydrated orthophosphoric acid and sulphuric acid and (10) sodium tripolyphosphate and sulphuric acid. The polymers may be dissolved in solvents such as dimethyl formamide, dimethyl acetamide, ethylene glycol carbonate, propylene glycol carbonate and concentrated salt solutions, and spun into filaments and fibres. Specification 867,006 is referred to.