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 .