Electron-transfer reactions at Ta electrodes covered by a thin (similar to 2.5 nm) native oxide film, hereafter designated as Ta/Ta2O5, have been investigated using scanning electrochemical microscopy (SECM). Oxidation and reduction of soluble redox species (I-, Fe(CN)(6)(4-), and Ru(NH3)(6)(3+)) are localized to randomly-distributed, microscopic sites (radius = 2-50 mu m) at the surface of the Ta/Ta2O5 electrode. The electroactive sites display an unusual dependence on the identity of the redox species. Specifically, SECM images show that some sites are only active for reduction of Ru(NH3)63＇ while others, on the same surface, are active for both the reduction of Ru(NH3)(6)(3+) and the oxidation of I-. The rate of I-oxidation at individual sites has been quantified by SECM and shown to approach the theoretical mass-transport-limited value, suggesting that the oxide film is either absent or very thin (similar to 1 nm) at these electroactive sites. The influence of the Ta2O5 film thickness on electron-transfer kinetics at individual sites has also been investigated. A sudden decrease in the electron-transfer rate is observed at individual sites during controlled anodic growth of the film. The abrupt decrease in electron-transfer rate at different redox-active sites occurs at electrode potentials spanning a similar to 1 V range, reflecting differences in the local energetics and kinetics associated with nucleation and growth of the Ta2O5 film.