The kinetics of the oxidation of mercury(I) by thallium(III) in aqueous perchloric acid solution, i.e. Hg(I)2 + Tl(III) → 2Hg(II) + Tl(I), have been examined. The rate law was found to be of the form −d[Hg(I)2]/dt = kexp[Hg(I)2][Tl(III)]/[Hg(II)] where kexp is inversely dependent on the concentrations of H+ and of ClO4−. The rate-determining step of the reaction appears to be a 'two-electron transfer' between a mercury atom, formed by the dismutation of Hg2++, and a hydrolyzed thallium ion, i.e. Hg + TlOH++ → Hg++ + Tl+ + OH−. The rate constant, k, of this reaction is given by k = 1016±2 exp[−14000 ± 3000/RT] liters mole−1 sec.−1.H+ retards the reaction by opposing the hydrolysis of Tl+++, while the effect of ClO4− appears to be due to its complexing with Hg2++, Cl− and Br− catalyze the reaction probably by complexing with Hg++, thus displacing the Hg2++dismutation equilibrium, [Formula: see text], to the right and increasing the concentration of Hg atoms. The kinetics and mechanism of the Tl(I)–Tl(III) isotopic electron exchange reaction and of other electron transfer processes in solution are considered in the light of these observations.