The reaction of 2,4-TDI and DEA, as an A(2) + B*B-2 polymerization system towards hyperbranched HPUs was followed using in situ ATR-FT-IR spectroscopy. The decrease in intensity of the NCO absorption band of the reactive isocyanate group of 2,4-TDI along with the formation and growth of the new characteristic bands of urethane and urea groups were detected. The reactivity difference of both NH and OH groups towards the NCO group at low temperatures was proven. The rate of the reaction was found to be affected by changing the temperature, the rate of addition of the B*B2 monomer and the type of solvent. Moreover, the increase of the carbonyl vibration and the amide II bands urea was very obvious during the addition of the stopper DEA. Thus, it was possible to verify the individual reaction steps of this complex polyreaction and to correlate these with the structural development of the resulting macromolecules.