The present invention relates to new polyols modified by phosphonoformamidine groups, to a process for the preparation thereof and the use thereof as starting component for the preparation of preferably foamed polyurethane plastics.The process involves (A) reacting diisocyanatocarbodiimides corresponding to the following general formula:OCN--R.sup.2 --N.dbd.C.dbd.N).sub.m R.sup.2 --NCOat a temperature of from about 25 to 130.degree. C., optionally in the presence of an inert solvent, with polyhydroxyl compounds corresponding to the following general formula:A--OH).sub.nin an OH/NCO-ratio of from 1.2:1 to 30:1 to form a polyhydroxyl compound containing carbodiimide groups and (B) completely or partially reacting the carbodiimide groups remaining in the product of step (A) by means of a basic catalyst at a temperature of from 25 to 150.degree. C., optionally in the presence of an inert solvent, with a phosphite corresponding to the following general formula: ##STR1## to form phosphonoformamidine groups, and (C) distilling off the inert solvent, if used; whereinm represents a number of from 1 to 10, andR.sup.1 represents a C.sub.1 -C.sub.18 aliphatic, C.sub.4 -C.sub.15 cycloaliphatic or C.sub.6 -C.sub.15 aromatic or araliphatic radical which may optionally contain a double bond and/or be branched and which may optionally be substituted by a group selected from the group of hydroxyl or trifluoromethyl groups, fluorine, chlorine, bromine or iodine;R.sup.2 represents a divalent aliphatic, cycloaliphatic, aromatic or araliphatic radical having from 4 to 25 carbon atoms obtained by removing the isocyanate groups from a diisocyanate;A represents an n-valent radical of the type obtained by removing the hydroxyl groups from a polyhydroxyl compound having a molecular weight of from 62 to 6000; andn represents an integer of from 2 to 8.