The invention relates to a process for the in situ production of dispersions or solutions of urea group-containing aromatic isocyanates by the low temperature reaction of aromatic diisocyanates and/or polyisocyanates (the NCO groups of which preferably exhibit different reactivities) with from 0.15 to 0.4 mols of water per NCO equivalent in relatively high molecular weight polyols and in the presence of catalysts accelerating the NCO/water reaction. Such catalysts are preferably selected from peralkylaminoalkyl ethers, peralkylamino-polyalkylene ethers and/or peralkyl-polyalkylene-polyamines. The present invention also relates to the resulting metastable solutions or dispersions and to the use thereof, optionally with the further addition of relatively high and low molecular weight compounds having hydroxyl, amino or hydrazide groups and with the optional addition of other polyisocyanates in polyurethane-forming ratios for the production of polyurethanes.