The glass transition temperature of an ether-based polyurethane shape memory polymer (SMP) has been found to decrease significantly after immersion in water. In order to get a better understanding of the mechanism behind this phenomenon, a systematic study on the effects of moisture on the glass transition temperature and thermomechanical properties of this SMP was carried out. The results reveal that the hydrogen bonding between N-H and C=O groups is weakened by the absorbed water. Furthermore, the water absorbed into the SMP can be separated into two parts, i.e. the free water and the bound water. Each part inside the SMP was quantified. Bound water significantly reduces the glass transition temperature in an almost linear manner and has a significant influence on the uniaxial tensile behavior, while the effect of free water is negligible. In addition, the recovery stress and recovery strain in constrained/free recovery induced by water were investigated and compared with that induced thermally. (c) 2005 Elsevier Ltd. All rights reserved.