The present invention relates to a process for the preparation of aqueous dispersions or solutions of isocyanate polyaddition products characterized in that oligourethanes(a) which have an average molecular weight of less than about 15,000,(b) which contain from about 0 to 120 milliequivalents, per 100 g of solid content, of built-in ionic groups and/or from about 0 to 25% by weight, based on the total weight of the oligourethane, of built-in ethylene oxide units --CH.sub.2 --CH.sub.2 --O-- present within a polyether chain and/or from about 0 to 30% by weight, based on the total weight of the oligourethane, of external emulsifiers which are not chemically fixed, the total quantity of the aforesaid hydrophilic groups or external emulsifiers being sufficient to ensure the dispersibility or solubility of the oligourethanes in water, and(c) which contain primary or secondary amino end groups or semicarbazide end groups with at least one NH end groupare reacted in the aqueous phase with a chain lengthening agent which is at least difunctional and which undergoes an addition reaction or condensation reaction with the amino end groups or semicarbazide end groups to effect chain lengthening of the oligourethanes. The present invention also relates to the dispersions or solutions obtainable by this process and to their use for the production of films and coatings.