Kinetic experiments on the extraction of Cr(VI) with Aliquat 336 performed in hollow fiber modules and in the 1% (v) less than or equal to Aliquat 336 less than or equal to 10% (v) and 10 g/m(3) less than or equal to Cr(VI) less than or equal to 500 g/m(3) range of concentrations have been successfully modeled by means of the integration of the mass conservation equation and associated boundary conditions. Different values of the mass transport coefficient through the organic membrane have been obtained depending on the total carrier concentration.