FIELD: chemical industry, more particularly preparation of fillers for plastic materials, paper rubber mixes, fibers and textile compositions. SUBSTANCE: black is treated with diazonium salt, stirred, filtered, washed and dried. Treatment is carried out in aprotic or protonic reaction medium, e.g. in water alcohol or the like. Diazonium salt is prepared beforehand or during treatment by reacting primary amine with nitrite and acid or aqueous solution of nitrogen dioxide. Primary amine has general formula: AyArNH2 wherein Ar is aromatic and heteroaromatic radical, A is substituent on said radical, e. g. functional group, linear, branched hydrocarbon radical; y>1. If y >1, A is the same or different. Carbon is treated in granulator by air-stream or water-stream granulation. The resulting product is essentially black and organic group chemically bonded therewith. Depending on composition of organic group, product shows different properties. EFFECT: more efficient preparation method. 115 cl, 12 tbl
