An on-line monitoring approach for froth stability in flotation processes has been developed based on electrical impedance spectroscopy. The electrical impedance spectrum (EIS) of the froth phase has been found to be sensitive to the froth structure and characteristics. The froth structure variation or fluctuation caused by bubble coalescence and rupture can be sensed by the variation or fluctuation of electrical impedance spectrum in the froth phase. The EIS electrodes can monitor the froth stability at real flotation conditions in a nearly non-invasive manner. An approach in which a stability index (SI) has been defined, has been developed to quantify the stability of the froth structure. A froth phase having a low SI value has been found to be stable and the corresponding operating conditions will be favourable in terms of the separation efficiency of flotation processes. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.