Chemical mechanical polishing (CMP) of tantalum was performed using alumina and fumed silica particles dispersed in chemical environments that are relevant to Cu CMP. The highest tantalum polish rate was obtained with the abrasives dispersed in deionized water and, in all cases investigated here, the polish rate decreased with the addition of the oxidant. Rotating disk electrode studies showed an increase in the open-circuit potential with time, saturating eventually, in the absence of abrasion indicating the formation of a passive film. Nevertheless, in situ electrochemical measurements (with abrasion) show that the open-circuit potential is lower during polishing. This indicates that the passive film is being removed, at least partially, during polishing exposing a less noble surface.