The effect of thermal history on the rheological behavior of ester- and ether-based commercial thermoplastic polyurethanes (TPUs) was investigated. H-1 and C-13 nuclear magnetic resonance (NMR) spectroscopy indicated that the ester-based TPU consisted of 4,4＇-diphenylmethane diisocyanate (MDI) and butanediol (BDO) as hard segments and poly(tetramethylene adipate) as soft segments, and the ether-based TPU consisted of MDI-BDO as hard segments and poly(oxytetramethylene)as soft segments. The dynamic storage and loss moduli (G＇ and G ") of specimens, which had been prepared by injection molding at different temperatures, were monitored at a fixed angular frequency during isothermal annealing. It was found that injection molding temperature employed for specimen preparation had a profound influence on the variations of G＇ and G " with time observed during isothermal annealing. Measurements were taken of the N-H stretching absorption bands in the Fourier transform infrared (FTIR) spectra during isothermal annealing at 170 degrees C for specimens prepared by injection molding at different temperatures. The analysis of FTIR spectra indicated that variations of hydrogen bonding with time during isothermal annealing resemble very much variations of G＇ with time during isothermal annealing. Isochronal dynamic temperature sweep experiments indicated that the TPUs exhibited hysteresis effect in the heating and cooling processes. It is concluded that the microphase separation transition or order-disorder transition in TPU cannot be determined from the isochronal dynamic temperature sweep experiment. It was observed that plots of log G＇ versus log G " varied with temperature over the entire range of temperatures (110-190 degrees C) investigated, suggesting that the morphological state of the TPU specimens varied with temperature. Little evidence was found from H-1 and C-13 NMR spectroscopy that exchange reactions took place in the TPU specimens during isothermal annealing at elevated temperatures.