A flexible polyurethane foam fire-retarded with 7.8% tris(1,3-dichloro-2propyl) phosphate (TDCPP) was found to lose by volatilization 80% of this material at 200 degrees C, a temperature at which there is as yet little decomposition of the foam. It is therefore to be expected that when this foam burns, most of its TDCPP will vaporize into the flame. The latter would therefore be expected to be the main site of activity of this retardant. However, when TDCPP was injected directly into the flame of the burning unretarded foam, no inhibiting effect was apparent. This, in contrast to the above conclusion, points to the condensed phase as the main site of the retarding activity. Although there is some additional evidence that supports the latter interpretation, it is difficult to reconcile it with the small amount of residual TDCPP available for reaction in the condensed phase. Flames of unretarded foams were also unaffected by the injection of other halogen containing materials, such as trichloropropane, HCl, and HBr. It is suggested that the temperatures of these polyurethane flames may not be sufficiently high to initiate the radical-trapping-based flame-poisoning mechanism classically attributed to the halogenated fire retardants. This could be the reason why the latter are less effective as fire retardants for polyurethanes than they are for many other substrates.