Diethyl vinyl phosphonate has been free-radically copolymerized with styrene, methyl methacrylate, acrylonitrile and acrylamide to give copolymers containing roughly equal amounts of covalently bound phosphorus. The flammabilities of these copolymers have been assessed, principally by measurements of limiting oxygen index, and compared with those of the corresponding homopolymers : polystyrene, poly(methyl methacrylate), polyacrylonitrile and polyacrylamide. It has been found that all the copolymers are more flame retardant than the corresponding homopolymers, especially the copolymer with acrylamide which appears to display considerable nitrogen-phosphorus synergism.