Process for the preparation of polyisocyanates containing allophanate groups

A process for the preparation of allophanate polyisocyanates having at least three isocyanate groups by the reaction of hydroxyl compounds with excess quantities of organic polyisocyanates which are free from allophanate groups, characterized in that the hydroxyl compounds used contain compounds represented by the following formula ##STR4## wherein R.sup.1 and R.sup.2 are identical or different and represent hydrogen or hydroxyalkyl, alkyl or cycloalkyl groups wherein at least one of the groups R.sup.1 and R.sup.2 must be hydrogen or a hydroxy alkyl group, and

• R.sub.3, R.sub.4, R.sub.5 and R.sub.6 are identical or different and represent hydrogen or alkyl or hydroxy alkyl groups or the groups R.sub.3 and R.sub.5 together with the two carbon atoms may form the basic structure of a cycloaliphatic ring.