The electrochemical reduction of carbon dioxide at copper in a hydrogen carbonate electrolyte saturated with CO2 was investigated using differential electrochemical mass spectroscopy and improved experimental and data processing techniques. The poisoning effects were investigated and it is shown that the deactivation of the copper cathode could be decreased by application of anodic pulses to the cathode and/or by addition of Cu2+ ions into the electrolyte. A soluble CO2 reduction product of yet unknown structure with a very low vapor pressure was found in the electrolyte. The accumulation of this compound led to a remarkable decrease of the activity of copper electrodes. It is confirmed that methane and ethene are the main reduction products on the copper cathode. A new product which gave a mass signal of m/e = 31 was detected. The activity of methane and ethene production and the stability of the copper electrode are influenced by the nature of the alkali ion in the hydrogen carbonate electrolyte. The course of deactivation as a function of the nature of the alkali ion as well as impedance measurements give information about the mechanism of the electrochemical CO2-reduction at copper cathodes.