Abstract: A method of treating foil for use in electrolytic capacitors in which the foil moves in one direction through a treatment liquid and in which the capacitance is measured continuously by means of at least one current-carrying electrode from which alternating current is conveyed and at least one voltage electrode at a small distance from the foil. The foil capacitance is measured by means of a phase-sensitive voltmeter. The measured capacitance is used to control the treatment method, etching or forming. In a preferred embodiment two current-carrying electrodes are present on either side of the foil and two voltage electrodes are present between the current-carrying electrodes and the foil on either side thereof. This avoids contacting of the foil.