The flow characteristics in hollow fiber modules (HFMs) were studied using resident time distribution (RTD) curves, which showed that the actual flow was nonideal and conformed to neither the ideal plug flow nor the complete mixed-flow models. Axial dispersion was found to decrease with an increase in flow velocity on the tube side and to increase with an increase in flow velocity on the shell side. Good agreement between the curves calculated using an axial diffusion model and experimentally determined RTD curves showed that the diffusion model can be used to describe the mass transfer characteristics in HFMs. In the study using 20% trioctylamine + 30% octanol + 50% kerosene-sulfanilic acid-water as actual extraction process, it was found that axial dispersion was the cause of the significant decrease in mass transfer performance. The resistance to mass transfer is located mainly in the boundary layer of aqueous phase. The large amount of treatment suggested that sulfanilic acid removal from its wastewater by HFM was an efficient process.