Tris(1,3-dichloro-2-propyl) phosphate (TDCPP) is used commercially as a fire retardant for flexible polyurethane foams. It was found to act both in the condensed (pyrolysis zone) and the vapor(flame) phases. The extent of its activity in the individual phases depends on the way in which the specimen is ignited. Under conditions of candle-like, top-down burning, retardation seems to occurs mainly in the condensed phase by a mechanism apparently based largely on the barrier properties of a phosphorus-containing carbonaceous layer that builds up on top of the liquid pyrolyzing layer beneath the flame. As the formation of this barrier requires time, extinguishment in this mode is relatively slow. Extinguishment is much faster in bottom-up burning, where the flame appears to be the main site of the retardation. In this mode, because of the orientation of the specimen relative to the flame, a disproportionately large amount of TDCPP enters the flame. It is proposed that TDCPP decomposition products, HCl and other low-fuel-value materials dilute the fuel vapors sufficiently to reduce their flame propagation velocity to below that at which they stream out of the pyrolysis zone. This pushes the flame away from the pyrolysis zone, uncoupling the thermal feedback mechanism that produces the fuel.