The oxidation of TcO2 with O2 in the presence of H2O at 250 °C was investigated; Tc2O7 and a red product were observed after the reaction. UV/Visible spectroscopy and a cerium titration on the “red” product are consistent with the presence of a reduced Tc species. It is proposed that the “red” product was formed by the hydrolysis of Tc2O7 to HTcO4 followed by the reduction of HTcO4. The reaction products of TcO2 with O2/H2O at 250 °C were also studied by mass spectrometry. The main dinuclear species were Tc2O7 and Tc2O5, while the main mononuclear species were TcO3 and HTcO4. The compound HTcO4 and associated hydrates were simulated to identify candidate structures for the “red” product; however, neither the acid nor any related hydrates exhibited the UV/Visible signature of the “red” product.