Permeation time lag in polymer hollow fiber membrane is investigated with the dual mode sorption isotherm and the partially immobilized diffusion model. Analytical solutions are derived for the time lag at the two limit cases while numerical solutions are provided for the general case. It is shown that curvature ratio plays an important role for the permeation time lag in polymer hollow fiber membranes at intermediate pressure range. (c) 2006 Elsevier B.V. All rights reserved.