Abstract: Flame-retardant fibrous material is prepared by reacting acrylonitrile polymer fibrous material with a guanidine compound of the formula ##STR1## where X and Y each represent hydrogen or an amine group, or a salt thereof, in a substantially water-free polar organic solvent in which the guanidine compound is soluble. The fiber produced incorporates both repeating diaminotriazine rings of the general formula ##STR2## dependent from the nitrile groups of the polymer chain and repeating groups of the general formula ##STR3## formed by cyclization of the polymer chain. The fibers can be rendered electrically conductive and used to form woven, non-woven or knitted fabrics.