Gold(III) halides catalyze the oxidation of sulfides to sulfoxides in a phase-transferprocess. The organic sulfides, dissolved in nitromethane, are treated with (Bu4N+AuCl4−) in catalytic amount and aqueous nitric acid which acts as oxidant. The oxidation of the thio-group is selective and can be carried out also in the presence of other oxidizable groups, such as vinyl, tertiary amino, hydroxy, diol