The present invention relates to a new process for the partial carbodiimidization of the isocyanate groups of organic mono-, or di- and/or polyisocyanates, the mixtures which can be obtained by this process and the use of these mixtures as isocyanate components in the production of polyurethane plastics by the known isocyanate polyaddition process. In general, the process comprises:(a) mixing organic mono-, di- and/or polyisocyanates with 0.1 to 100 ppm of a phosphorus compound of the general formula ##STR1## wherein R represents a lower alkyl radical, a phenyl radical, an alkoxy radical, a hydrogen atom or an alkenyl radical; a, b, c and d each represent hydrogen, a halogen atom, a lower alkyl, lower alkenyl, phenyl, a cyclohexyl or a polymethylene group which together with two neighboring C-atoms of the heterocyclic ring forms a cycloaliphatic ring; and X represents an oxygen or sulphur atom; at a temperature from about 0.degree. to about 200.degree. C., and,(b) terminating the carbodiimidization reaction after the evolution of 1 to 90% of the quantity of carbon dioxide to be theoretically expected in the carbodiimidization of all isocyanate groups present, by the addition of a quantity at least equivalent to the phosphorus compound used of a compound which deactivates the catalysts while forming an adduct therewith.