Polyurethane (PU) foams were protected by phosphorous and nitrogen-based molecular flame retardant hexamethoxycyclotriphosphazene c-[N = P(OMe)2]3 (HMCPT). This compound was synthesized, analyzed, and investigated with respect to its thermal behavior, compatibility, and efficiency as a flame retardant (FR) for PU foams. The decomposition of HMCPT starts at 186 °C in air and under argon, as indicated by TG/DSC measurements. It was possible to introduce 5 per hundred parts polyol (phpp) of HMCPT into the PU foams. The mechanical and morphological characteristics of the protected foams—compression load deflection, tenacity, tensile elongation, air perviousness, and raw density—were investigated indicating that the FR has an impact on all of these properties. The flame retardancy was evaluated with the FM VSS 302 test, where an self-extinguishing classification was reached with 5 phpp of HMCPT. Cone calorimeter measurements provide evidence for a flame poisoning mechanism. Copyright © 2013 John Wiley & Sons, Ltd.