The mechanism of catalysis of aromatic bromination by pyridine was investigated by measuring the rates of bromination of mesitylene in acetic acid and in chloroform with and without added pyridinium or other substituted ammonium salts. Most salts increased both the second- and third-order rate constants for bromination, and pyridinium salts were no more effective than others. Bromide salts decreased the rate and sometimes the order of the bromination reaction by complexing bromine as tribromide. The formation constants of pyridinium tribromide and tetra-n-butylammonium tribromide in chloroform at 25.0° were found to be 2 × 102and 9 × 104 M−1respectively. It is concluded that in dilute solution pyridine catalysis of bromination is simply a salt effect which can be accounted for by the commonly accepted mechanism of bromination.