Abstract: A process is described for the dimerization of a substituted aromatic compound of the formula (I): ##STR1## wherein X represents --OH, --SH or --NHR where R is H, an alkyl group or an aryl group; R.sup.1 and R.sup.2 represent an alkyl group, an aryl group, an alkenyl group, an alkynyl group, an alicyclic group, a halogen atom, an alkoxy group, an aryloxy group, a hydroxy group, or an amino group; and R.sup.3 and R.sup.4 can have the same definition as R.sup.1 and R.sup.2 and may additionally represent a hydrogen atom; which comprises reacting the substituted aromatic compound in the presence of a peroxidase enzyme, a peroxide and a radical transfer agent in an aqueous medium.