Aqueous polyurethane anionomer dispersions were prepared from isophorone diisocyanate, poly(tetramethylene adipate glycol) (PTAd) and dimethylol propionic acid as potential anionic centres. The effects of polyol molecular weight (M(n)) on the state of dispersion, thermal, mechanical and viscoelastic properties and swelling of emulsion-cast film were determined. With increasing M(n) of PTAd, the particle size of the emulsion and soft segment glass transition temperature, T(g), decreased, and solvent swelling, emulsion viscosity and hard segment T(g) increased. Tensile strength showed a minimum with PTAd 1000, and elongation at break generally increased with the M(n) of PTAd. These results could be interpreted in terms of soft segment-hard segment phase separation and crystallization of high molecular weight PTAd.