Solute permeability and reflection coefficients (sigma) through hollow-fiber membranes were measured for cellulose and poly (methyl methacrylate) (PMMA) membranes. The dialysis apparatus used was a U-type hollow fiber unit which was loosely bundled and set in a perfectly mixed reservoir with dialyzate flowing perpendicularly to the hollow fibers. Mass transfer resistance across the boundary layer outside the fibers was small at low How velocity, so solute permeability could be measured accurately. The mass transfer correlation obtained was Sh=0.38Re(out)0.72Sc1/3 Ultrafiltration experiments were performed for the case of flow within the fibers, in order to determine actual rejections and sigma values. The relationship between sigma and the molecular weights of polyethylene glycols (PEGs) in the cellulose membrane approached that of inulin and other solutes when the amount of hydrated molecules of water to PEG units was taken into account. Negative sigma values were observed in PMMA membrane for PEGs #1000-#6000.