The phase transition behavior of a commercial thermoplastic polyurethane (TPU) is investigated by rheological techniques. Oscillatory shear flow experiments are performed during both the isothermal annealing following a rapid cooling from the melt state, and the subsequent heating ramp (remelting). Rheological measurements reveal that the microphase separation between hard and soft segments and the concurrent crystallization of the hard phase domains produce a sol-to-gel type of transition. We show that the microstructure of TPUs at the critical gel point is strictly related to the applied thermal history. In particular, the critical gel properties during the cooling down transition are found to be quantitatively different from those measured during the reverse, heating up process. When coupled to differential scanning calorimetry measurements under the same thermal history, the theological technique is shown to be a useful tool to determine the mechanisms of microstructure evolution. (C) 2004 The Society of Rheology.