Polystyrene and poly(methyl methacrylate) copolymers with cyclophosphazene side groups have been prepared by the reactions of azidocyclophosphazenes with diphenylstyrylphosphine residues in the copolymer structures. The cyclophosphazene pendent groups were present in from 1 to 100% of the monomer residues in polystyrene and from 2 to 20% of the repeating units in poly(methyl methacrylate). In both systems, the cyclophosphazene side units lowered the T,roughly in proportion to their concentration in the macromolecules. The presence of similar to 10% of cyclophosphazene-bearing monomer residues markedly reduces the combustibility of both polystyrene and poly(methyl methacrylate). The mechanism of fire retardance is discussed.