Three types of water based polyurethane (PU), i.e. one ordinary elastomeric polyurethane (EPU) and two thermosensitive polyurethane (TSPU), were synthesized and applied in leather finishing. Differential scanning calorimetry, positron annihilation lifetimes (PAL), water swelling, and water vapor permeability (WVP) were measured to evaluate how the structure of the synthesized PU and the temperature influenced the WVP of the PU finished leather. In contrast to EPU, TSPU(b) with a glass transition reversible phase and TSPU(c) with a crystal transition reversible phase showed an obvious phase separation structure and a phase transition in the normal wearing temperature range. PAL study indicated that when the temperature was higher than the designed phase transition temperature, the ortho-positronium lifetime (tau(3)) and the average radius (R) of free volume of TSPU showed dramatic changes, whereas tau(3) and R of EPU remained unchanged. The water swelling and WVP of TSPU finished leather were found to depend on the structure of the polymer and the temperature, and they gave different responses to temperature variation. When the temperature was higher than the designed phase transition temperature, a significant WVP increase from 3800 g/(m(2) 24 h) to 7830 g/(m(2) 24 h) for TSPU(b) finished leather and from 4100 g/(m(2) 24 h) to 9450 g/(m(2) 24 h) for TSPU(c) finished leather were observed. Whereas EPU finished leather showed low WVP, and increased slightly as temperature increased. Phase transition accompanying a significant change in WVP can be used to develop "smart leather" with controllable water vapor permeability. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 1820-1827, 2010