Forced-adiabatic, FT i.r. spectroscopy has been employed to simultaneously monitor polymerization and microphase separation on model flexible polyurethane foam systems. The following combinations of hydroxy functional components were investigated : (1) polyether-polyol and water; (2) polyether-polyol and deuterium oxide; (3) polyether-monol and water; and (4) polyether-monol and deuterium oxide. The formation of urethane, soluble urea, soluble D-urea, hydrogen-bonded urea and associated D-urea species were monitored during their fast bulk copolymerization with a diisocyanate. The decay of isocyanate is correlated to the polymerization kinetics and the evolution of hydrogen-bonded urea/associated D-urea is analysed emphasizing the onset of microphase separation of urea hard-segment sequences. The microphase separation transition (MST) occurred at a critical conversion of isocyanate functional groups. In the deuterium oxide blown foams, there was no trace of hydrogen-bonded urethane in the spectra obtained. In the polyether-monol systems, a lower conversion of isocyanate was observed at the MST and an overall lower reaction conversion was also observed.