AbstractThe limiting oxygen indices of flame‐retardant treated wools have been determined at environmental temperatures up to 380°C. As the ambient temperature increases, the LOI decreases significantly until, within the temperature range of 308°C and 325°C, it reaches a minimum value, Above 325°C, the LOI of the carbonaceous residue increases rapidly with temperature. The minimum value reached for a number of retardant treated wools was below that for untreated wool. Broadly similar behaviour is exhibited by flame‐retardant cotton, modacrylic and Nomex°, although with the latter there is no increase in oxygen index at the upper end of the temperature range