To develop a novel microporous membrane that responds to moisture/liquid content for use in protective clothing, the surface of microporous polyurethane membrane was modified by graft polymerization with different molecular weights of poly(ethylene glycol) (PEG). Surface grafting was confirmed, and appropriate grafting time and temperature were determined. The hydrophilicity of microporous polyurethane membranes was improved after surface modification with PEG. Both the surface pore sizes of the PU membranes and the constricted parts of through pore sizes of the bulk PU membranes were reduced, as revealed by the pore size distribution using image analysis and capillary flow porometer, respectively. The reduced pore sizes are expected to enhance the barrier properties by reducing the possibility of harmful particles and liquid borne pathogen penetration. Water vapor transmission rate (WVTR) measurements indicated that the pores of the modified PU membranes were responsive to the moisture content. (c) 2005 Elsevier B.V. All rights reserved.