A process for the preparation of stable aqueous polycarbodiimide dispersions to be used as cross-linking agents, in which initially an isocyanate functional polycarbodiimide is prepared from a polyisocyanate and a mono-or polyisocyanate which contains a hydrophobic group. Thereafter the polycarbodiimide chain is capped and/or extended by reaction of the isocyanate functions or a part thereof witha hydrophilic amine- or hydroxy functional compound and of the remaining isocyanate functions with an amine- or hydroxy functional compound which contains hydrophobic groups, after which the obtainedproduct is dispersed in water and the pH is adjusted to 9-14. The hydrophobic groups are hydrocarbons with 4-25 carbon atoms, fluorinated hydrocarbons, silicone functional hydrocarbons or polysilicones . Further, the invention relates to a coating mixture in which the polycarbodiimide dispersion is used as cross-linking agent and to the cured material obtained with the coating mixture.