Ta and Nb are group V valve metals which resist corrosion. Anodic dissolution of Ta and Nb in acidic fluoride media of varying HF concentration is investigated using potentiodynamic polarization and electrochemical impedance spectroscopy. Polarization curves showed a clear active and passive region in all the solutions employed in this study. At a given HF concentration, Nb anodic polarization currents are larger compared to those of Ta. EIS of Ta and Nb exhibited a low frequency capacitive loop in active region, and a low frequency negative differential resistance in the passive region. The surface of Ta and Nb was characterized using XPS, and it reveals the presence of both sub oxide and pentoxide. The electrochemical results could be explained by a four step mechanism involving sub-oxide and pentoxide intermediates. The analysis shows that direct dissolution of sub-oxide occurs via an electrochemical pathway and is facilitated by HF2- species and HF in undissociated form. On the other hand, chemical dissolution of pentoxide, which occurs in parallel, is facilitated by the HF species in undissociated form. (C) The Author(s) 2018. Published by ECS.