The Polarographie behaviour of Hg++ ions in a solution of AlBr3 and KBr in toluene or mesitylene (Trimethylbenzene) has been studied. All chemicals have been dried and purified by standard vacuum techniques and experiments were performed inside a glove box, in an atmosphere of ultra-high purity argon. Simple Polarographie behaviour was observed only in very dilute solutions (less than ca. 0.25 mM). The diffusion coefficient in this region was calculated to be (1–2) × 10-6 cm2/s-1 in mesitylene (depending on the concentration of the electrolytes) and 4 × 10-6 cm2/s-1 in toluene. At higher concentrations catalytic currents and a Polarographie maximum are observed and the limiting current is proportional roughly to the square of the concentration. Chemical dissolution of mercury was found to occur in this system. This is fastest in benzene and slowest in mesitylene. As the concentration in solution is increased the open circuit potential changes according to the Nernst equation.