Morphology development during reaction-induced phase separation in flexible polyurethane (PU) foam has been studied by time-resolved, small-angle X-ray scattering. Two different PU foaming systems, based on industrial formulations, have been studied. The scattering data have been analyzed using the Cahn-Hilliard model and fit to a Teubner-Strey microemulsion structure factor to extract morphological characteristics. Microphase separation was observed to follow spinodal decomposition kinetics; a bicontinuous morphology developed and was subsequently preserved by vitrification of the urea domains. The data analysis demonstrated that the amphiphilicity factor increases as the reactions proceed, due to the continuous formation of block copolymer poly(ether-urea).