A process as well as an immersion sensor for measuring an electrochemical activity of a layer lying on a melt is provided, using an electrochemical sensor, which has a measuring cell and a counter electrode. In order to make possible reliably reproducible and accurate measurements in the layer, the measuring cell and counter electrode are first immersed in the melt, wherein the measuring cell and counter electrode are protected from contact with the layer and wherein the measuring cell and counter electrode are brought into contact with the melt and are heated. After that, the measuring cell is pulled up to perform the measurement in the layer, wherein the counter electrode is located in the melt during the measurement. For this purpose, the measuring cell and the counter electrode have a protective cover, and the measuring cell is arranged, in the immersion position of the sensor, above the counter electrode.