Thermal-induced simultaneous phase separation and liquid-solid transition (gelation) in waterborne polyurethane dispersions has been detected morphologically and theologically. The viscoelastic material functions, such as dynamic shear moduli, G' and G" complex shear viscosity, eta* and loss tangent, tan 5 were found to be very sensitive to the structure evolution during the gelation process and the subsequent formation of a fractal polymer gel. At the onset temperature of the gelation process, an abrupt increase in G, G" and eta* (several orders of magnitude) was observed during the dynamic temperature ramps (2 degrees C/min heating-rate) over a wide range of angular frequency. The temperature dependencies of G, G" and tan delta were found to be frequency independent at the gel-point, T-gel, providing a fingerprint for determining T-gel of the dispersions. Furthermore, a dramatic increase in zero-shear viscosity, eta(0) (v-shape) was observed at T=T-gel and found to be in good agreement with the value obtained from the tan 5 versus T data. As expected, the time-temperature-superposition principle was found to be only valid for temperatures lower than the T-gel; the principle failed at T >= 70 degrees C. The morphology of the dispersions at 70 degrees C for 2 h showed for 36, 38 and 40 wt% formation of a network structure having a unique periodicity and phase connectivity. A lower critical solution temperature (LCST) phase diagram was estimated based on the different morphologies of the dispersions. The coexistence of liquid-liquid and liquid-solid transitions at the same temperature range confirmed the complex behavior of the polyurethane dispersions, pointing to the need for a new theory that explicitly takes this special behavior into account. (c) 2005 Elsevier Ltd. All rights reserved.