Phase separation in segmented copolyureas and copoly(urethane-urea)s formed by reaction injection moulding (RIM) has been investigated by studies of the formation of model polyurea-polyether blends with no covalent bonds between the phases. The polyblends have similar thermal and small-strain mechanical properties to the analogous copolymers. Removal of 80% of the polyether by selective solvent extraction indicates the continuity of the polyether phase. Electron micrographs of the isolated polyurea show a random-continuous polyurea structure with a wavelength of ≈ 200 nm. The experimental data are interpreted in terms of the polymerization being equivalent to a thermodynamic quench from one-phase space into two-phase space. Thus the co-continuity of the phases is characteristic of the early stages of spinodal decomposition and has been retained by vitrification of the polyurea.