A hydroxyalkyl or aminoalkyl ester of a phospholane phosphonic or phosphinic acid of the formula ##STR1## in which R.sup.I is alkyl with 1 to 10 carbon atoms or aryl with up to 14 carbon atoms,R.sup.2, r.sup.3 and R.sup.4 each independently is alkyl with 1 to 4 carbon atoms, hydrogen, chlorine or bromine,R.sup.5 and R.sup.6 each independently is alkyl with 1 to 8 carbon atoms and, where a or b = 0, also an aryl radical with up to 8 carbon atoms and, where a = 1, also I/m of an m-valent cation or a hydroxy alkylene or imino alkylene radical of the formula ##STR2## R.sup.7 and R.sup.8 each independently is hydrogen, optionally substituted alkyl with 1 to 6 carbon atoms, optionally substituted phenyl, alkenyl with 2 to 4 carbon atoms, chloromethyl, bromomethyl, hydroxy methyl or etherified or esterified hydroxy methyl, in addition to which R.sup.7 and R.sup.8 may together be an alkylene bridge of 2 to 4 carbom atoms to form a ring,R.sup.9 and R.sup.10 each independently is hydrogen or alkyl with 1 to 2 carbon atoms,Z is oxygen or imino,n is a number from 1 to 30,X and Y each independently is oxygen or sulphur, anda and b each independently is 0 or 1, with the proviso that at least one of a and b is 1 and at least one of R.sup.5 and R.sup.6 is one of the above-mentioned hydroxy alkylene or imino alkylene radicals,Is produced by reacting a phospholane phosphonic or phospholane phosphinic acid of the formula ##STR3## with at least one alkylene oxide or alkylene imine of the formula ##STR4## The products are useful for converting isocyanates into carbodiimides.