Small-angle x-ray scattering (SAXS) and transmission electron microscopy (TEM) have been applied to study polymer morphologies of CO2-blown rigid polyurethane foam samples of varying isocyanate index. The results are consistent with an irregular (meso) phase segregated structure with phase boundaries showing fractal symmetry. Phase segregation persists throughout the index range, although the interfacial surface tends to smoothen with increasing isocyanate index to eventually result in a significant degree of energetic ’sharing’ across the phase boundary. The latter can satisfactorily be explained by a free volume double layer (FVDL) model. Mechanistically, fractal symmetry was attributed to the competition between crosslinking and phase segregation tendencies in rigid PU foam polymers. The resulting frozen-in morphology represents an early stage of the development towards a more regular spinodal phase segregation, as occurring in polyurethane foam systems of lower crosslink density (e.g. flexible foams).