Processes and devices are disclosed for continuously detecting at least one substance in a gaseous or liquid mixture by means of a sensor electrode to which is applied a variable potential. In an alternative, the substances are concentrated at the surface of the sensor electrode, their concentration is determined by measuring the electrode capacity, the thus obtained measurement value is correlated with the substance and the substance. concentrated at the surface of the sensor electrode is then removed. In another alternative, one or several detection cycles are carried out. In each detection cycle, at least one substance is concentrated at the surface of the sensor electrode, the potential is brought to at least one potential characteristic of the electrochemical reaction of at least one substance, the resulting current is measured and the thus obtained measurement values are correlated with the substances.