The invention relates to a process for preparing arylsulphonyl isocyanates of the general formula (I) ##STR1## and addition derivatives thereof--where in the formula the meaning of the substituents are as follows:Ar represents optionally substituted aryl, naphthyl- or thienyl group, andHX is a chemical bond or XZR.sub.1 stands for a group where Z is oxygen or sulphur,R.sub.1 is saturated or unsaturated alkyl, benzyl, cyclohexyl or substituted phenyl group orX is an NR.sub.2 R.sub.3 groupwherein R.sub.2 R.sub.3 is hydrogen, alkyl, unsubstituted alkyl, aryl, substituted aryl, aroyl or acyl group, and the essence of the process is that an N-halogen sulphonamidate is carbonylated in the presence of a carbonylating catalyst, in the presence or absence of alcohol, (thio) alcohol or amine reactants and the carbonylated compound obtained is reacted subsequently with the reactants.