In this study, metal ions were extracted from aqueous solutions across a hollow fiber containing carriers in kerosene, and the metals were then back-extracted through another hollow fiber to a stripping phase. Two extraction systems of Cu2+-LIX64N-HCl and Cu (EDTA)(2-) - Aliquat 336-HCl were studied. A mass transfer model was developed which took into account possible steps, such as aqueous layer diffusion, interfacial chemical reaction, membrane diffusion, and organic layer diffusion. It was found that this model could acceptably predict the time profiles of aqueous concentrations of metal ions in such modules (standard deviation, 11%). Based on this mass transfer model, a procedure was employed with pseudo-steady-state approximation to numerically determine the fractional resistance of each transport step. This enabled us to identify the mechanism during the extraction and back-extraction processes. In the Cu2+-LIX64N-HCl system, the resistance of aqueous layer diffusion increased and of interfacial reaction decreased with time under high extractant concentrations. In the Cu(EDTA)(2-)-Aliquat 336-HCl system, the resistance of organic layer diffusion decreased and of aqueous layer diffusion increased with time under high Cu(EDTA)(2-) concentrations. However, the same trends of resistances with time were observed in the back-extraction module.