Abstract: The present invention relates to a process for preparing a low boiling monoisocyanate having a boiling point of between 70 and 320° C. at standard pressure by reacting A) a high boiling isocyanate compound having a boiling point of at least 180° C. at standard pressure and an HC content of at least 50 ppm with B) a monoamine having a primary amino group, at a maximum reaction temperature of 180° C. and a molar ratio of isocyanate groups to amino groups of at least 4:1 to form a compound containing a biuret/urea group, simultaneously thermally decomposing this compound in situ to form a monoisocyanate corresponding to monoamine B) and removing the monoisocyanate by distillation, optionally under vacuum.