Abstract: The invention relates to a sensor for detecting substance concentration or activity or for determining the presence of substances based on electrochemical reactions. The electrochemical sensor comprises an electrode with surfaces inside the electrode, wherein electrochemical detection reactions occur. Said sensor guarantees high flow densities and is suitable for miniaturization. A substance-recognizing agent is advantageously placed in the electrode in contact with the inner surfaces. This enables not only short response times and long service life but also measurements with excellent linearity.