A compound adapted to be used to advantage as a catalyst for making carbodiimide foams is prepared by a process whereinA. a phosphetane oxide or phosphetane sulphide of the general Formulae I and II: ##EQU1## wherein X represents a C.sub.1 -C.sub.18 alkyl, C.sub.2 -C.sub.18 alkenyl, C.sub.6 -C.sub.14 aryl, C.sub.7 -C.sub.20 aralkyl, C.sub.1 -C.sub.18 alkoxy or C.sub.6 -C.sub.14 aroxy group andR.sup.1 to R.sup.6 represent hydrogen and/or C.sub.1 -C.sub.4 alkyl groups and is reacted withB. a mono-, di- and/or polyhydric alcohol having a molecular weight of about 32 to about 250, a protonic acid which has a pH of between 1 and 8 in N/10 aqueous solution, a metal salt or an acid chloride.