Abstract: Reactor for carrying out electrochemical reactions with an electrode and a counter-electrode are known, whereby the electrodes are separated form one another by a capillary gap. Such reactors do not comprise a homogeneous flow rate distribution in the capillary gap which can thus lead to a reduction of selectivity and of local temperature increases. In addition, high ohmic resistance requires the use of conducting salts. The aim of the invention is to provide a reactor in which the voltage drop in the electrolyte is minimized and which can be better optimized over a short resistance time distribution. To this end, the capillary gap comprises at least one channel as a reaction chamber having a height less than or equal to 200 &mgr;m. According to the inventive method, the volume flow of the educt current is measured such that the flow of the educt current is laminar over the length of the channel having a height less than or equal to 200 &mgr;m.