Abstract: Process for preparing flame retarding polyurethane foams from a polyhydroxyl compound, diisocyanate mixture, a blowing agent, a catalyst and any other auxiliary agent if necessary, comprising mixing polyhydroxyl compound having at least two active hydrogen atoms per molecule, a molecular weight of 1,000 to 10,000 and wherein at least 10% by weight of the hydroxyl groups are primary hydroxyl groups, and a diisocyanate mixture containing 10 to 60% by weight of a modified diisocyanate prepared from the reaction of a diisocyanate compound expressed by the formula ##STR1## wherein X is an oxygen or sulfur atom.