The present invention is directed to aqueous solutions or dispersions which may contain blocked polyisocyanates of cationic oligourethanes which are crosslinkable at elevated temperature and contain(a) about 0.1 to 3% by weight of ternary or quaternary ammonium ions introduced through Michael adducts of secondary amines having at least one hydroxyl group and specific acrylic acid derivatives,(b) up to about 2.8% by weight of tert. amino nitrogen atoms incorporated through Michael adducts of secondary amines having at least one hydroxyl group and specific acrylic acid derivatives and(c) about 0 to 15% by weight of blocked isocyanate groups, provided that the ternary ammonium groups of (a), the tert. nitrogen atoms of (b) and the blocked isocyanate groups are present in an equivalent ratio of (a+b):blocked isocyanate groups of about 1:1 to 1:1.5. The present invention is additionally directed to a process for the preparation of these solutions or dispersions of oligourethanes and to their use for coating heat resistant substrates, particularly by coating metallic substrates by cathodic electrodeposition and subsequently cross-linking these coatings at elevated temperature.