Crosslinking reaction of hydroxyl-terminated polybutadiene (HTPB)/isophorone di-isocyanate (IPDI) was monitored by infrared spectroscopy, dynamic mechanical analysis (DMA), swelling measurements, and by relaxation time (T 2 ) measurements obtained by low-field NMR technique. Chemical reaction monitored by FTIR shows that urethane bonds were predominantly formed throughout the whole reaction period while DMA and swelling became only effective once the three-dimensional network was formed. NMR results allow differentiating between relaxation-processes associated with different fractions of the reactants in the mixture prior to the network formation. The most important finding in this study is that two of the relaxation processes were found to decline whereas a new fraction with a short relaxation time which emerged specifically at an early stage of reaction and progressed along with advancement of the reaction. All results pointed out to a change in the mixture behavior around 30 h of crosslinking reaction at 60 C, reflecting an important restriction in molecules diffusion and mobilities which were attributed to the gel point formation.