In order to improve a method for determining the substance conversion during areal electrochemical reactions in at least one local surface area between a counterelectrode arrangement of an areal design and an electrode arrangement of an areal design, which has a contact element segment designed in accordance with the surface area and a contact element contacting the remaining surface areas and being electrically insulated in relation to the contact element segment, in such a manner that the substance conversion can be determined in as simple a manner as possible it is suggested that not only the contact element but also the contact element segment be connected to one current source or current drain provided for carrying out the electrochemical reaction, that a current flowing between the contact element segment and the current source or current drain via a conductor generate a magnetic field around the conductor and that the magnetic field be determined as a measure for the substance conversion in the local surface area.