The equilibrium concentrations in membrane contactors are investigated in closed system for the extraction of alkali metal picrates with the 1,3-bis-benzo-crown-6-calix[4]arene in toluene (95%)-decanol (5%) in order to understand the extraction kinetics through the membrane. The modeling of the equilibrium distribution curves, which is carried out from the mass action law, allows the determination of the reaction equilibrium constant (K-ex). The presence of alkali chloride (0.1 M) in the initial aqueous phase changes the equilibrium concentrations: K-ex is 1.5 x 10(6) L-2 mol(-2) for the cesium picrate extraction with cesium chloride while it is 3.75 x 10(6) L-2 mol(-2) in the absence of cesium chloride. The calculation of the distribution coefficient (D) in the membrane contactor is carried out from global mass balances (operating line) and non-linear extraction isotherms. The experimental distribution coefficients are in agreement with the calculated values. The high ionic strength and hydrophilic character of chlorides in the aqueous phase lead to an increase of the picrate transfer and to a considerable increase of the distribution coefficient from 0.45 (C-aq(degrees)= 1.48 X 10(-4) mol L-1) for the cesium picrate extraction without cesium chloride to 170 (C-aq(degrees) = 1.7 x 10(-4) mol L-1) when cesium chloride is present. On the other hand, it appears that for high initial concentration in the aqueous phase (C-aq(degrees) > 7.0 x 10(-4) mol L-1 for the cesium picrate) and in the presence of alkali chloride occurs the saturation of the organic phase with calixarene complex. This organic phase saturation inside the membrane pores generates a precipitate that slows down the transport kinetics. (C) 2008 Elsevier B.V. All rights reserved.