Limiting (LOI) and extinction (EOI) oxygen index values have been determined for combustion modified high resilience polyurethane foams of a range of densities. The data obtained show that the value of the extinction oxygen index at zero ignition time, (EOI), is less than the LOI value. (EOI) is directly related to the flame retardant concentration of the foam sample rather than foam density which is the determining factor of the LOI values. Thus the (EOI) parameter can be considered to be a superior measure of the intrinsic foam flammability. General conclusions are drawn as to the relationships between foam density and melamine flame retardant content and the LOI and (EOI) values.