Abstract: A process for the preparation of aromatic compounds of the general formula I,Ar--X Iin which Ar represents an optionally substituted aryl group and X represents a group OH, Cl, Br or CN, by reaction of an aryl diazoniumsalt of the formula II,Ar--N.dbd.N.sup.+ Y.sup.- IIin which Ar is as defined above and Y represents a suitable counterion, with water, hydrogen chloride or a salt thereof, hydrogen bromide or a salt thereof, or a cyanide in the presence of a copper(I)salt, characterized in that the copper(I)salt is a copper(I)salt derived from an hydroxy carboxylic acid.