For an understanding of the four electron transfer mechanism of the electrochemical oxygen reduction reaction (ORR), a direct detection of intermediates on well-defined working electrode surfaces is crucial, as it bridges the gap between electrochemistry and computation. Here, the ORR is studied on the crystalline semiconductor n-Ge(1 0 0) by electrochemical attenuated total reflection infrared (ATR-IR) spectroscopy in aqueous NaClO4 and NaCl solutions at pH 10.5. Germanium-bound superoxide at a tilt angle of similar to 45 degrees with respect to the surface normal is found as an intermediate in both electrolytes. The frequency of the superoxide O-O stretching mode differs by 25 cm(-1) between the two electrolytes, hinting at a complex formation between at least one of the anions and the superoxo-intermediate. This complex formation may be related to the different surface terminations of germanium in the two electrolytes. The surface concentration of the superoxide intermediate is proportional to the ORR current. Analysis of the relation between absorbance and current shows that the rate constant of the second electron transfer step in the ORR, i.e. the step from superoxide to peroxide, in NaCl is three times as high as in NaClO4. (C) 2013 Elsevier Ltd. All rights reserved.