The anodic dissolution of aluminium in electrolytes in which alkyl- mono- or polyhalides are present together with aromatic compounds, gives rise to the in situ formation of the well known Friedel-Crafts catalysts, promoting the chemical alkylation of the aromatic ring with the halide. A chemical reaction dissolving aluminium is activated by the electrochemical one, and , after the passage of a limited amount of charge, becomes predominant. At this stage the process becomes strongly exothermic, cannot be controlled by electrochemical means and proceeds until one of the reagents, aluminium or the halide, is almost completely consumed. The resulting “red oils” are good catalysts even for alkylations in which halides different from the starting one are employed.