This article reports the application of Fourier Transform Infrared-Attenuated Total Reflectance (FTIR-ATR) technique for investigation of in situ polymerization of polyurethane-urea elastomers. Formulations comprising of diisocyanates, a polyether diol and a diamine based chain extender have been used in this study. The diisocyanates used were 4,4'-diphenylmethane diisocyanate and toluene diisocyanates. The polyether diol and diamine used were propyleneglycol block-PEO-b-PPO and 4-(4-(2-(4-(4-amino-2-(trifluoromethyl) phenoxy) phenyl) propan-2-yl) phenoxy)-3-(trifluoromethyl) benzeneamine, respectively. These reactants were mixed and placed on the ATR cell, and then the infrared spectra were recorded at an interval of 1.75 s while continuously heating. The polyurethane-urea formation was monitored by the decay in the intensity of isocyanate band at 2258 to 2261 cm(-1). As the polymerization progressed, new peaks appeared at wavenumbers of 1711 cm(-1), 1697 cm(-1), and 1655 cm(-1). These peaks correspond to the urethane carbonyl, hydrogen bonded urethane, and urea carbonyl groups, respectively. It was found that with the progress of the reaction, the shift in the peaks at 1655 and 1697 cm(-1) occurs gradually. This shift in peaks is attributed to the hydrogen bonding. The hydrogen bonding and hence the shift in the peak is a cumulative effect of three phenomena namely: (1) degree of polymerization, (2) macro and microphase separation, and (3) temperature effect. A rationale is discussed to deconvolute these three effects. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 1012-1018, 2011