Muhammad Zakir, Tsutomu Sekine, Tsutomu Takayama, Hiroshi Kudo, Mingzhang Lin, and Yosuke Katsumura Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan Nuclear Engineering Laboratory, School of Engineering, University of Tokyo, Ibaraki 319-1106, Japan Received: July 31, 2005; In Final Form: September 30, 2005 Mechanisms of the radiolytic formation of 99TcIV O2¥nH2O colloids were studied by bremsstrahlung irradiation of an aqueous Pertechnetate (TcVIIO4-) solution under different conditions; pH of the target solution, concentration of TcO4and presence or absence of radical scavengers and of seed particles of TcO2•nH2O. Colloid particccccticcles (30 130 nm) of TcO2¥nH2O a coagulate of nanoparticles with the size of 2 nmwere radiolytically produced at pH higher than 3, whereas a soluble Tc(IV) species but colloid was formed at lower pH. The soluble Tc(IV) species transformed to the TcO2¥nH2O colloid when the pH of the solution was raised to 3 or higher after the irradiation. This fact suggests that the soluble Tc(IV) species is .a precursor of the TcO2•nH2O colloid. The presence of TcO2•nH2O seed particles in the target solution of TcO4did not enlarge of the partcile size, revealing that the solidification would take place rather in the solution than on the surface of nanoparticles. The reduction of TcO4in acidic solutions proceeded mainly through processes involving a bimolecular reaction of TcO4with hydrogen radicals, followed by the successive disproportionation reactions of Tc(VI) and Tc(V).