Solute permeability and reflection coefficients (σ) through hollow-fiber membranes were measured for cellulose and poly(methyl metharrylate) (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 flow velocity, so solute permeability could be measured accurately. The mass transfer correlation obtained wasSh = 0.38Reout0.72Sc1/3 Ultrafiltration experiments were performed for the case of flow within the fibers, in order to determine actual rejections and σ values. The relationship between σ 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 σ values were observed in PMMA membrane for PEGs #1000–#6000.