Plastics and elastomers were prepared, physical state determined by the side chains attached to the phosphorus–nitrogen backbone. The poly(aryloxyphosphazenes) displayed a high degree of flame retardancy in the uncured, unfilled state. Limiting oxygen index (LOI) values varied from 27 to 33 for nonhalogenated materials and from 38 to 65 for halogenated materials. These values qualify all poly(aryloxyphosphazenes) studied as flame retardants according to the generally accepted definition (LOI ≥ 27). Materials subjected to the National Bureau of Standards Smoke Test gave encouraging results. The [(C6H5O)2PN-(4-C2H5C6H4O)2PN]n elastomer was investigated for application as a fire retardant insulating foam. Closed-cell foams produced from this copolymer showed much improved fire retardancy and smoke generation compared to commercially available fire retardant foam insulations.