Demethylation of 3′, 4′-dimethoxy or 3′, 4′, 5′-trimethoxy benzoic ester of a phenol is carried out in the presence of an excess of aluminum halide in an organic solvent to get 4′-Hydroxy, 3′-methoxy or 4′ hydroxy, 3′, 5′-dimethoxy benzoic acid ester of a phenol. The reaction is also applicable to 3′, 4′, 5′-trimethoxy diaryl ketone and some natural products like reserpine.