Tantalum (Ta) electrochemistry is studied in aqueous HF and NH4F by cyclic voltammetry and electrochemical impedance spectroscopy. The much greater hydrogen evolution current and the much lower charge-transfer resistance for the high-frequency impedance loop in HF relative to NH4F demonstrate that the native Ta2O5 film dissolves in HF, but not in NH4F. Weight loss experiments and cyclic voltammetry demonstrate that at cathodic potentials in aqueous HF, dissolution of Ta oxide allows electrochemical dissolution of the exposed surface into Ta5+. In addition, unusual low frequency impedance behavior is seen for Ta in HF analogous to that reported by Bojinov for several different metal surfaces at anodic potentials in concentrated acids. This suggests the presence of an ultrathin, passive film in HF and is consistent with the poor adhesion often obtained for Cu electrodeposition onto Ta. This passive film may correspond to the Ta suboxide (TaO) previously observed by X-ray photoelectron spectroscopy at the Ta/Ta2O5 interface. © 2005 The Electrochemical Society. All rights reserved.