This work has been focused to the determination of the membrane mass transport coefficient in the simultaneous non-dispersive extraction and back-extraction of Cr(VI). Following the methodology previously reported by the authors for the kinetic modelling of hollow fiber extraction and back-extraction processes, considering the assumption that the overall mass transport resistance is dominated by the resistance in the organic membrane, when Aliquat 336 is used as carrier. and after the description of the interfacial chemical equilibrium in the extraction process (non-Linear model, K-eq = 0.35) and in the back-extraction process (linear model, H-r = 3.6), it has been possible to describe successfully the experimental results obtaining the optimum value for the membrane mass transport coefficient, K-M = 2.2 X 10(-8) m/s, which allows the design and optimization of the recovery of Cr(VI).