We have investigated the reaction mechanism of the electrochemical reduction of carbon dioxide to hydrocarbons on copper electrodes. This reaction occurs via two pathways: a C₁ pathway leading to methane, and a C₂ pathway leading to ethylene. To identify possible intermediates in the reduction of carbon dioxide we have studied the reduction of small C₁ and C₂ organic molecules containing oxygen. We followed the formation and consumption of intermediates during the reaction as a function of potential, using online mass spectrometry. For the C₁ pathway we show that it is very likely that CHO_ads is the key intermediate towards the breaking of the C–O bond and, therefore, the formation of methane. For the C₂ pathway we suggest that the first step is the formation of a CO dimer, followed by the formation of a surface-bonded enediol or enediolate, or the formation of an oxametallacycle. Both the enediol(ate) and the oxametallacycle would explain the selectivity of the C₂ pathway towards ethylene. This new mechanism is significantly different from existing mechanisms but it is the most consistent with the available experimental data.