High-functionality polyisocyanates are prepared by a process which comprises (i) preparation of an addition product (A) which contains one group which is reactive toward isocyanate and at least two isocyanate groups by reacting (a) a diisocyanate or polyisocyanate I with (b1) compounds having at least three groups which are reactive toward isocyanate or (b2) compounds containing two groups which are reactive toward isocyanate or mixtures of (b1) and (b2), where at least one of the components (a) or (b) has functional groups having differing reactivities toward the functional groups of the other component and the reaction ratio is selected so that the addition product (A) contains an average of one group which is reactive toward isocyanate, (ii) if desired, intermolecular addition reaction of the addition product (A) to form a polyaddition product (P) containing an average of one group which is reactive toward isocyanate and an average of more than two isocyanate groups, and (iii) reaction of the addition product (A) and/or the polyaddition product (P) with a diisocyanate or polyisocyanate II.