A phosphazene derivative, hexa-allylamino-cyclotriphosphazene (HACTP), was synthesized through the aminolysis of hexa-chloro-cyclo-triphosphazene with excess allylamine. HACTP is a soluble light-brown crystalline solid, which melts at 90degreesC. It was reactive after melt but decomposed at 236degreesC. When it was incorporated into the resin formulations of unsaturated polyesters, it acted as a reactive flame retardant. The oxygen index was increased from 20.5 to 25.2, rating from the flammable to self-distinguishable scale. (C) 2003 Wiley Periodicals, Inc.