Using di-(2-ethylhexyl) phosphoric acid (D2EPHA)/cadmium ion/water as an experimental system, the mass transfer characteristics in hollow fiber modules (HFMs) were studied by Voronoi tessellation method based on normal distribution function, in which the standard variance could be correlative with the packing irregularity of fibers. The theoretical results showed that not only the packing density but also the packing irregularity significantly influenced the mass transfer characteristics in HFMs. For modules with the same densities, the different packing irregularity led to different mass transfer efficiency. The efficiency of modules with moderate packing density was the lowest because of their most serious packing irregularity. Compared with the exponent distribution function used in previous studies, the normal distribution function can determine the effect of packing irregularity on mass transfer performance accurately. The experimental results also showed a high extraction efficiency of cadmium ion from the wastewater by HFMs. (C) 2003 Elsevier Ltd. All rights reserved.