The phase transition of a series of thermoplastic polyurethanes has been investigated by means of theological and calorimetric techniques. Five different samples of varying molecular weight (MW) and hard-to-soft segment ratio have been studied. Thermal histories included isothermal annealing after cooling from the melt and heating cycles following isothermal annealing. The coupled use of the two different techniques allows for a quantitative description of the kinetics of phase transition and for a good understanding of the corresponding microstructural changes. At least for the family of polymers here investigated, such a process is a combination of two distinct stages, a faster one due to the activation of hard segment interactions, and a subsequent slower one, related to the microphase separation between soft and hard segments. A decrease of MW is found to moderately accelerate the phase transition kinetics, whereas an increase of the hard segment fraction shifts [he phase transition to higher temperatures. When the critical gel behavior is considered, experiments clearly indicate the equivalence of either decreasing MW or increasing the hard segment content. Some practical implications of the combined effect of MW, hard segment fraction and phase transition temperature on the polymer processing behavior are also discussed. (c) 2005 The Society of Rheology.