Liquid-phase mixing in a cold-flow counter-current-flow bubble column reactor with trays has been evaluated. A developed filtering technique [P. Gupta, M.H. Al-Dahhan, M.P. Dudukovic', P.L. Mills, A novel signal filtering methodology for obtaining liquid-phase tracer responses from conductivity probes, Flow Meas. Instrum. 11 (2) (2000) 123-131.] was used to extract the liquid-phase concentration responses from the conductivity probe data in two phase gas-liquid flow with systematic corruption. Conductivity probe responses to a pulse input of KCI liquid tracer were measured at various point-wise locations in an 8 in. diameter clear acrylic column using air and water as the gas-liquid system. The superficial gas and liquid velocities were selected to span those encountered in a commercial unit using a pipe-type gas sparger with a number of laterals. Results from tracer experiments are presented along with a cross-correlation of signals to estimate the characteristic residence time of the liquid on a tray. The utility of the filtering algorithm for a broader range of gas-liquid flows is also discussed. (c) 2006 Elsevier B.V. All rights reserved.