Various embodiments include a CO2 electrolyzer comprising: a gas space adjoining a cathode comprising a gas diffusion electrode adjoining a cathode space; an anode in an anode space; a membrane separating the cathode space from the anode space; a feed apparatus feeding reactant gas into the gas space; a mixing vessel for at least partial joint accommodation of the anolyte and the catholyte; and a connection line between the gas separating region and the gas space. The mixing vessel includes a gas separating region closed off with respect to a surrounding atmosphere. The cathode space accommodates a catholyte and the anode space accommodates an anolyte.