Structure development during reaction injection moulding of a copoly(isocyanurate-urea) was studied using time-resolved, synchrotron SAXS. During the rapid copolymerization of liquid reactants, incipient microphase separation was shown to occur at a critical conversion of isocyanate groups and to proceed via the kinetics associated with spinodal decomposition. Microphase separation was halted prematurely by vitrification of the polyisocyanurate phase, thus producing a copoly(isocyanurate-urea) with a nonequilibrium, co-continuous morphology with a size scale of approximate to 100 Angstrom.