To understand the behaviour of Tl/II/ and the bridging group SO
in radiation induced electron exchange reactions we have investigated the rate constants and mechanisms of reaction of Tl/II/ with Tl/I/ and Tl/III/ in perchloric acid solutions. The results indicated that Tl/II/ is an intermediate in the -ray induced electron exchange process of T1/I/–T1/III/. Sulfate ions at [SO
]>-0.02M serve as bridging groups and play an important role in accelerating the T1/II/–T1/I/ reaction. A cooperative effect was found between hydrogen peroxide and sulfate ion at low sulfate ion concentration, [SO
]0.02M in perchloric acid solution.