The UV–visible spectral characteristics of the Ag, Tl and bimetallic Tl/Ag clusters stabilized by polyethylene glycol and produced by radiation-induced reduction using both gamma and electron irradiation, have been studied in aqueous medium. A comparison of the two reduction methods, suggested that the high dose rate conditions of electron irradiation resulted in intimately alloyed mixed clusters with almost no aggregation at higher dose. Whereas, under the lower dose rate conditions of gamma irradiation, the bimetallic clusters with core of Ag and shell of Tl were formed due to the reduction of Tl⁺ions over the initially produced Ag nanoparticles. Only the thallium part of the mixed clusters were oxidized by both methyl viologen (MV²⁺) chloride and oxygen, leaving behind absorption due to Ag nanoparticles.