Determination of the transport properties of hollow fiber membranes contained within modern hemodialyzers is indispensable for assessing device design and optimizing the hemodialysis process. To measure these transport properties, a new and simplified experimental system employing dialyzer mini-modules was developed. In comparison to a full-size dialyzer, the advantages of using mini-modules for experimental purposes are: (1) each hollow fiber is assured to be patent; (2) interaction among the fibers is negligible. Mini-module dimensions were 15 cm (length) and 1.2 cm (diameter) and each device contained 10 hollow fibers. A special syringe pump was used to ensure the steady flow in the experiments. We applied this system to measure hydraulic permeability (L-p), the diffusive permeability (P-s) and reflection coefficient (a) of four solutes (urea, creatinine, vancomycin and inulin), for several different types of hollow fiber membranes. These determinations allowed mean pore sizes to be calculated, and these measurements were in close agreement with estimates obtained by SEM image analysis. This approach provides fundamental transport property data necessary for optimal membrane and, ultimately, dialyzer design. (c) 2005 Elsevier B.V. All rights reserved.