The development of corrosion-resistant waste forms suitable for long-term, safe disposal of Tc-99 is an important objective for the back-end of advanced nuclear fuel cycles. In this work, atomistic modeling has been employed to provide fundamental information about the durability of proposed Tc-containing materials. Specifically, first-principles studies have been carried out to investigate the reactivity of hydrogen, oxygen, and water on Tc(0001) and MoTc(111) surfaces. Our findings indicate that the preferable adsorption sites on Tc(0001) and MoTc(111) surfaces strongly depend on the adsorbates. The calculated oxygen adsorption energy on Tc(0001) is in excellent agreement with previous studies. Water adsorption on the Tc(0001) and MoTc(111) surfaces has also, been elucidated. The Eh-pH diagram of the Tc-O-H system has been constructed by solving the Nernst equations for possible reactions between Tc and its oxidizing products and compared to experimental data. The modified embedded atom method has also been employed to simulate the response of the MoTc(110) surface to active dissolution as a function of the Tc content in the alloy. It is found that the addition of small amounts of Tc (up to 10%) lowers the corrosion rate under these conditions. (C) 2013 The Electrochemical Society. All rights reserved.