The paper deals with mass transfer in liquid-liquid extraction in both directions using the following ternary systems, water-acetone-chloroform and water-acetone-toluene. The experiments were performed in an improved Lewis cell operating in continuous mode, made out of two baffled compartments equipped with separate driven agitators. Each compartment contains a stirred or non-stirred phase. The transfer of the solute can be carried in the two directions through an interface of constant area, created between the two phases. The specific resistances on mass transfer were determined in the stirred or non-stirred phases, with the influence of the partition coefficient and transfer direction on these resistances being also evidenced. (C) 2002 Elsevier Science B.V. All rights reserved.