Methods for the incorporation of phosphate and phosphonate units into the side groups of phosphazenes, at both the high polymeric and cyclic trimer levels, are described and evaluated. Halogenated organophosphates were used as the phosphorylating agents and were allowed to react with hydroxyl-or lithio-functionalized phosphazenes. Well-defined phosphorylated cyclic trimers were synthesized from the hydroxyl-functionalized precursors, and these were examined as name-retardant additives to polystyrene. The thermal stabilities of the high polymers made by this route were analyzed by TGA.