Abstract: The invention relates to a method for determining an operating state of an electrochemical system (1a; 1b; 1c; 1d; 1e), which has a cell stack (2) having at least one electrode portion (3, 4), at least one valve (5, 6, 20, 21) and at least one fluid line (23) being provided, the method comprising the following steps: conducting, in a varying manner, at least one fluid through the at least one valve (5, 6, 20, 21) via the at least one fluid line (23) with a predefined variation pattern, the variation pattern being applied to a fluid flow by means of the at least one valve (5, 6, 20, 21), determining a voltage response and/or a current response of the cell stack (2) during the varying conducting of the at least one fluid, and determining the operating state of the electrochemical system (1a; 1b; 1c; 1d; 1e) on the basis of the voltage response and/or the current response.