A cellulose triacetate (CTA) dialysis membrane is generally regarded as a homogeneous membrane. The objective of the present study was to characterize the internal structure of CFA dialysis membranes. We used FB-150E, FB-150F, and FB-150UH CIA hollow-fiber membrane dialyzers. The pore-size distribution and porosity of the CIA membrane surfaces were calculated from surface image data obtained by using atomic force microscopy (AFM). The internal porosity of the CIA membranes was measured by weight analysis. Further, the pore diameter of the CFA membranes was calculated from data on pure-water permeability and diffusive permeability using a pore diffusion model. The pore diameters on the inner surface of the CIA membranes were much greater than those on the outer surface. The surface porosity determined using AFM was much lesser than the internal porosity determined using weight analysis. The pore diameters on the inner and outer surfaces of the CTA membranes determined using AFM were much lesser than pore diameter calculated on the basis of the permeability data. We conclude that the CTA dialysis membrane, which is generally regarded as a homogeneous membrane, is actually a multilayer membrane. The CIA dialysis membrane has tight layers at the inner and outer surfaces, and has a loose layer inside the membrane. (C) 2010 Elsevier B.V. All rights reserved.