The method of at least one thermally expandable graphite intercalation compound, containing A) a least one metal halide of at least one of the elements Fe, Al, Sb, Zn, Y, Cr and Ni and B) at least one nitroalkane of the general formula CH.sub.3(CH.sub.2).sub.nNO.sub.2, in which n is a whole number from 0 to 10, as well as its structural isomers or mixtures, as intumescing, fire-protection additive in polymer matrices for producing intumescing fire-protective seals for through holes, wall bushings and other openings in walls, floors and/or ceilings of buildings, as well as a method for preparing the thermally expandable graphite-intercalation compounds used.