The procedure of preparation of a tin-containing modifier for cracking catalysts was developed. The process was conducted as a one-pot synthesis by reacting the tin metal with industrially produced C-7-C-9 synthetic fatty acids, with air bubbling through the reaction mixture. The application of the modifier increases the resistance of microspherical, zeolite-containing cracking catalysts to poisoning with the vanadium that is present in heavy petroleum feedstock.