ζ-potential measurements of vinylon fiber in the presence of polymer solutions were carried out by the stream compression method. The polymers used were polyethylene oxide (PEO), sodium polyacrylate (SPA) and Tororo-aoi mucilage (TA). The relation between the stream compression method and permeability equation which is the underlying principle of the method was discussed. The stream potential data in all solutions yielded the swollen specific volums of fiber, α, which were significantly greater than those calculated from permeability data. The discrepancy between the α values from stream and permeability methods was interpreted by the β-layer, in which partially dissolved molecules of fiber, bearing an electrical charge, extend into the liquid. Futhermore, the α values from both methods for vinylon in polymer solutions were greater than that in water. It is presumed that the large α values in polymer solutions arise from the non-Newtonian flow of the solutions , and that with viscosity correction both the stream compression and permeability methods are valid for the determination of ζ, α and surface area of fiber in polymer solutions.