A variant of ac impedance spectroscopy is applied to monitor and control electrochemical cells and appliances without need for reference values and knowledge of control points in advance. Electrolyzers, fuels cells, energy stores, sensors and electrochemical reactors are steered to an optimum operating state by continuous evaluation of capacitance and the derivatives thereof. Dry and humid electrode-electrolyte interfaces are distinguished with the aid of the low-frequency impedance. The problem is solved in order to determine electrolyte concentrations unambiguously from electrolyte resistance, although the conductivity of the solution has a maximum and changes nonlinearly with the concentration. (C) 2003 Elsevier B.V. All rights reserved.