This paper describes the dead-end ultrafiltration of humic substances with hollow fiber modules. We show that the resistance of a fouling layer decreases with increasing flux. The lower resistance of the fouling layer is caused by a loss in retention of the membrane for humic acid at these higher fluxes. The ideal cake filtration model does not fit the filtration resistance appropriately even considering a retention smaller than unity. The extended model falls short since it does not incorporate an axially dependent retention. Using five small modules in series, as well as a specially designed flat sheet cell, we prove that both the flux and retention in a fiber module is a strong function of the axial coordinate (fiber length). The experimental results with the small modules gave a complete picture of the axial features in dead-end filtration of humic substances with fibers, and the results can be used to explain the observed behavior in the large module. The main contributor of the axial phenomena is hypothesized to be charge interactions between the membrane and the solute combined with the small size of the foulants causing diffusive concentration gradients of the humic acids. (C) 2008 Elsevier B.V. All rights reserved.