The phenomenon of catalytic maximum has been investigated in the bromination of p-bromophenol (ArH) using iodine and anhydrous aluminium chloride as catalysts. The effective catalysts have been shown to be iodine bromide in acetic acid and carbon tetrachloride media, and AlCl3, Br2 in acetic acid. A correlation between the ratio of the reactants at the catalytic maximum, and the order in the effective catalyst has been deduced and experimentally verified. © 1969, CSIRO. All rights reserved.