AbstractTwo experimental methods have been developed to characterize the pressure‐driven flow of a liquid through the walls of single hollow‐fiber membranes. In one method, the permeation rate is measured directly by following the air–water interface in a pressurized pipet connected to a hollow fiber which is sealed at one end. With the second method, the permeation rate is determined by tracking the descent of an air bubble inside a pressurized hollow fiber sealed at one end. Results obtained by the two experimental methods are in good agreement. Darcy's law is used to analyze the experimental data because the Darcy permeability constant, k, is an intrinsic property of the material. The membrane dimensions and the Darcy permeability constant for reconstituted collagen hollow fibers are shown to be quite sensitive to hydrogen ion concentration and ionic strength. A conceptual link between the Darcy permeability constant and the membrane microstructure is obtained by combining the aligned‐rod structural model for reconstituted collagen proposed by Kramer with a hydrodynamic calculation due to Happel and electrical double‐layer theory. On the average, the predictions of this model for volume fraction solids and microstructure dimensions are in reasonable agreement with experimental observations.