Oxalic acid ͑OA͒-based nonalkaline solutions of H 2 O 2 are found to support chemically mediated removal of Ta-oxide surface films on Ta. The associated surface reactions are critical for chemical mechanical planarization ͑CMP͒ of Ta barrier layers in the fabrication of interconnect structures. In the present work, we probe the underlying mechanisms of these reactions using dc and ac electrochemical techniques. A Ta coupon electrode is used as a model system in abrasive-free solutions of 1 wt % OA + x wt % H 2 O 2 at pH 3.0 and 6.0 ͑x = 0, 1, and 5͒, where the chemical component of CMP is selectively examined in the absence of mechanical abrasion. These nonalkaline solutions are essential for preventing chemical damage to low-k dielectrics underlying Ta barriers in interconnects. The electrochemical data presented here explain the recently reported findings of Ta CMP experiments using OA solutions, and provide the necessary framework to put forward a reaction scheme for the chemically mediated disintegration of Ta 2 O 5 films formed on Ta in such solutions. A comparison of the dc and ac electrochemical data helps to identify the relative roles of the different chemical additives in these CMP solutions.