Spatially-localized electrochemical reactions at Ti electrodes covered by a thin (similar to 20 Angstrom) semiconducting oxide layer (Ti/TiO2) have been investigated in aqueous solutions using scanning electrochemical microscopy (SECM). Oxidation of iodide, bromide, and ferrocyanide is shown to occur at randomly positioned microscopic sites having characteristic diameters of ca. 50 mu m. The surface density of electroactive sites observed in SECM images is relatively low (20 +/- 5 sites/cm(2)), representing a minuscule fraction (0.04%) of the total electrode area. SECM of Ti/TiO2 electrodes in the presence of different redox species demonstrates that the spatial localization of electron-transfer reactions is dependent upon the potential at which the redox species is reduced or oxidized. Spatially-localized activity is observed for reactions occurring at potentials positive of the conduction band edge of TiO2, E(CB). The electroactive surface sites have metal-like properties, i.e., they are electrochemically active at potentials where the depletion layer structure of the oxide film should inhibit the electron-transfer reaction. In contrast, the reduction of ferricyanide at potentials negative of E(CB) occurs uniformly on the electrode surface, i.e., the heterogeneous electron-transfer reaction is spatially delocalized. The results demonstrate the unique capability of SECM for mapping spatial- and potential-dependent reaction rates at semiconductor electrodes.