This paper presented micro-structure, free volume and water vapor permeability of dense segmented polyurethane (SPU) membrane. Wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) techniques were employed to investigate the micro-structure. Free volume was measured by positron annihilation life spectroscopy (PALS). WAXD and DSC results indicated microcrystalline structure presents in the polymer membrane. Percent crystallinity of SPU would depend on the types of polyol used. The microphase-separated structure of copolymers was observed by transmission electron microscopy. Extent of phase separation was depended on the type and composition of polyol used. Free volume hole sizes are different for SPU samples due to the interaction between the polymer chains and packing of polymer chains due to the different compositions. The influences of temperature and chemical compositions on the free volume and water vapor permeability have been discussed. Experimental results revealed that the water vapor permeability can be controlled by free volume, which would depend on the chemical composition of SPU, and hydrophilicity of membranes. (c) 2006 Elsevier B.V. All rights reserved.