A series of polyester-based poly(urethane urea) (PUU) aqueous dispersions with well-defined hard segments were prepared from polyester polyol, 4,4 ' diphenylmethane diisocyanate, dimethylolpropionic acid, 1,4-butanediol, isophorone diisocyanate, and ethylenediamine. These anionic-type aqueous dispersions had good dispersity in water and were stable at the ambient temperature for more than 1 year. For these aqueous dispersions, the particle size decreased as the hard-segment content increased, and the polydispersity index was very narrow (< 1.10). Films prepared with the PUU aqueous dispersions exhibited excellent waterproof performance: the amount of water absorption was as low as 5.0 wt %, and the contact angle of water on the surface of this kind of film was as high as 103 degrees (this led to a hydrophobic surface). The water-resistant property of these waterborne PUU films could be well correlated with some crystallites and ordered structures of the well-defined hard segments formed by hydrogen bonding between the urethane/urethane groups and urethane/ester groups, as well as the degree of microphase separation between the hard and soft segments in the PUU systems. (c) 2005 Wiley Periodicals, Inc.