A new methodology has been developed to investigate the oxidation state and processing of tantalum (Ta) during an electrochemical mechanical polishing (ECMP) process. Polishing experiments under different mechanical forces were conducted at constant potential. Faraday’s law was used to calculate the material removal rate (MRR), which was compared with measurements using an atomic force microscope (AFM). It was found that there were multiple valences of Ta and unknown types of its oxides formed during ECMP. The Preston equation was tailored to the current polishing conditions, showing that the MRR was a linear function of a mechanical force. It was found that, during ECMP, metastable oxides of Ta were formed. With higher mechanical force, more Ta suboxides were formed, while with lower mechanical force, more Ta pentoxide appeared. Such a mechano-oxidation process, from suboxides to the pentoxide, was illustrated using a physical model where the transition from suboxides to the pentoxide dominated. The present research demonstrates an effective ex situ method to study the in situ oxidation process of Ta. The present approach can be applied to other materials as well, and used to help researchers to understand and optimize the ECMP process.