Dispersion-free extraction of thallium(III) from NaCl/H2SO4 solution into butyl acetate has been studied using hydrophobic polypropylene and polyvinylidene fluoride hollow fibers of different effective lengths. The aqueous feed solution flowed inside the fibers and the organic phase was fed at the shell side. The influence of phase flow rates on the mass transfer coefficients was investigated, and correlations between the Sherwood and Graetz numbers were developed. The overall resistance to mass transfer is controlled by the feed-side mass transfer coefficient due to the high partition coefficient of HTlCl4 between the phases. The obtained results are discussed in terms of extraction efficiency, thallium flux through the interfacial area, and the length of the transfer unit as a function of the feed flow rate and the effective fiber length. The extraction efficiency was independent of the organic phase flow rate.