The present invention relates to a polyisocyanate mixture containing isocyanate trimers provided thati) 30 to 100 mole percent of the trimers are iminooxadiazine diones B and ##STR1## ii) 0 to 70 mole percent of the trimers are isocyanurates A iii) less than 10 mole percent of the polyisocyanate mixture is of the uretone imine structural type, andiv) the ratio of the sum of the mole percents of trimers A and B to the mole percent of uretdiones is greater than 4:1whereinR.sup.1, R.sup.2 and R.sup.3 are the same or different and represent the groups obtained by removing an isocyanate group from an aliphatic, cycloaliphatic, aromatic and/or araliphatic isocyanate having an NCO content of less than 70% and/or their oligomers.The present invention also relates to a process for the preparation of these isocyanate trimers, mixtures of these isocyanate trimers with other polyisocyanates and compositions containing these isocyanate trimers, in which the isocyanate groups may optionally be blocked with blocking agents, and compounds containing two or more isocyanate-reactive groups.