In work related to the electrocatalysis of the CO2 reduction reactions, we recently reported in This Journal the structure and composition of a Cu(100) electrode surface, pre-dosed at low levels of O-2(g) to simulate a Cu electrocatalyst unprotected from air, before and after immersion in alkaline electrolyte at fairly negative potentials to ascertain if an oxide-to-metal reduction reaction can be effected; experimental measurements were based upon ex situ techniques, low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). It was found that the mildly oxidized surface remained ordered and could be cathodically reduced back to a well-ordered oxide-free Cu(100); the quality of the LEED pattern and AES spectrum was less than ideal, however, due to small amounts of base electrolyte remnant in the emersed layer. In this Short Communication, we present results from operando electrochemical scanning tunneling microscopy (EC-STM) that not only confirm the earlier observations but, more importantly, depict more accurately the actual electrocatalysis conditions. An as-received commercially oriented Cu(1 0 0) disk that had not been protected from air was observed to consist of narrow terraces encrusted with highly disordered oxides. Cyclic voltammetry and coulometry showed that the oxidized surface consisted of five monolayers of CuO and quarter of a monolayer of Cu2O. Upon complete cathodic reduction of the interfacial oxides, the surface was found to have reverted to a single-crystalline Cu(100)-(1 x 1) structure. It may thus be inferred that, under the conditions of alkaline electrochemical CO2 reduction, the Cu catalyst exists as a zerovalent metal. (C) 2014 Elsevier B.V. All rights reserved.