A method is described and tested experimentally to determine the profile of the radial volume flow density, the concentration profiles of an impermeable and permeable solutes along the axis of high-flux hollow fiber dialyzers as well as the extent of internal filtration. Dextran ( approximate to 5.5 x 10(6) g mol(-1); , mean molar mass) is used as an osmotically active impermeable solute and creatinine (M=113 g mol(-1)), vitamin B-12 (M=1335 g mol(-1)) and cytochrome c (M=12 384 g mol(-1)) as permeable solutes. These four solutes are present simultaneously in the aqueous solution passing the lumen of the dialyzers. The experimental data show that the transport of low molar mass solutes (M approximate to 100 g mol(-1)) across the membrane of a high-flux hollow fiber is dominated by diffusion. The transport of solutes of molar mass about M approximate to 10 000 g mol(-1) is influenced by both diffusion and convection. A published model is used to describe the experimentally determined concentration profiles of the impermeable and permeable solutes in the lumen of the fibers. The agreement between the prediction of the model and the experimental findings is satisfactory.