Bias is an important variable in column flotation operation: negative values will deteriorate the concentrate grade due to mechanical entrainment of gangue, whereas excessive bias will reduce the particle residence-time in the collection zone as well as dilute the tails stream. Its estimation is usually made, at a high instrumentation cost, through a water mass-balance in the collection zone, nonetheless providing unreliable estimates. This paper presents a new method based on the measurement of the volumetric fraction of wash-water underneath the pulp-froth interface, using electrical-conductivity sensors. The method has been evaluated in a pilot flotation column, with a two-phase system. The results obtained indicate that it can be directly used for supervision and control of flotation column operation. (C) 2008 Elsevier Ltd. All rights reserved.