In this work, benefiting the advantages of recycle systems, a new and simple explicit equation is proposed for the determination off overall mass transfer coefficients, K in continuous dialyzers. Under certain conditions (i.e. high fluid velocities through the dialyzer, low membrane area and/or low membrane permeability) the measurement of differences between the dialyzer inlet and effluent concentrations, so as to find mass transfer rate, can be very difficult and inaccurate. The present approach might be applicable for a wide range of operating conditions, and is free from the probable limitations of once through flow systems, since it relies on the monitoring of cumulated time change of reservoir concentration in a recycle system. Continuous dialyzer overall mass transfer coefficients were predicted with the employment of a single reservoir concentration and the corresponding time data in the proposed equation. (C) 2003 Elsevier B.V. All rights reserved.