Abstract: Black, flame-resistant, substantially inert, non-conductive, flexible textile materials are produced by impregnating a scoured regenerated cellulosic textile material with a composition containing (a) a compound having in one tautomeric form, the general formula ##STR1## in which X is oxygen, sulphur or --NH--, and Z is --NH.sub.2, --NHGN, --NH.NH.sub.2, --NH.CO.NH.sub.2 or lower alkyl or X and Z taken together represent a trivalent nitrogen atom, and (b) an ammonium salt of an oxy-acid of phosphorus, heating the impregnated textile in an oxygen-containing gas to a temperature of 220.degree.-300.degree. C. until a black, flame retardant product is obtained and there has been a loss in weight based upon the original weight of the textile material, of 20% to 30%.