In this study, a new equation for estimating the height of the mass transfer unit, H-oxp, for mutually insoluble extraction systems was developed. The equation's accuracy and robustness were tested by comparing the predicted results with experimental data in the literature. With the same apparent interfacial tension, the mass transfer unit height decreased hyperbolically with the increase of the overall interphase volume mass transfer coefficient, despite the differences of column dimensions, experimental systems and operating conditions. (C) 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.