Various waterborne interpenetrating polymer networks (IPNs) composed of polyurethaneurea (PUU) and graft vinyl ester resin were prepared from polyester polyol, dimethylolpropionic acid, isophorone diisocyanate, ethylenediamine and a well-defined graft vinyl ester resin containing butanol side chains (BO-g-VER). These anionic IPN aqueous dispersions were stable at the ambient temperature for >1 yr, and also had the excellent stability at low and high temperatures. The experimental results showed that the introduction of BO-g-VER network has not only greatly affected the particle morphology, stability and rheological behavior of these aqueous dispersions, but also rendered the films with the augmented surface hydrophobicity as well as the excellent water-resistance performance and mechanical properties. A synergistic effect was observed at a certain BO-g-VER composition (20 wt %) to give significant reinforcement to the PUU, resulting from the compatibility and/or the interpenetration between the PUU network and the BO-g-VER network. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 1070-1079, 2009