The pH is an important parameter in the reaction mechanism of the electrochemical reduction of carbon dioxide and carbon monoxide to methane and ethylene on copper electrodes. We have investigated the influence of the pH on this reaction using Cu(111) and Cu(100) single crystal electrodes. The results support our recently proposed reaction mechanism, in which two different reaction pathways to ethylene can be distinguished: a first, pH-dependent pathway that has a common intermediate with the formation of methane that occurs mainly on Cu(111), and a second, pH-independent pathway via a carbon monoxide dimer. The latter pathway occurs on Cu(1 00) only. (C) 2013 Elsevier B.V. All rights reserved.