The role of thermal history on the nanoscale segregated structure of a bulk polymerized polyurea containing oligomeric poly(tetramethylene oxide) soft segments is investigated in the present study. Temperature-dependent unlike segment demixing was explored in two series of experiments: at constant heating (and cooling) rate and on annealing at selected, elevated temperatures. Tapping mode atomic force microscopy on the as-polymerized polymer demonstrates that the polyurea hard segments self-assemble into a ribbon-like morphology that is generally preserved on annealing, although ribbon coarsening was observed at the highest annealing temperature. The results from the constant heating rate synchrotron X-ray scattering experiments demonstrate that the nanoscale structure begins to reorganize at temperatures as low as similar to 70 degrees C, and the very significant changes in mean interdomain spacing observed at much higher temperatures are largely retained on returning to ambient conditions. Although. there was surprisingly no detectable difference in the degree of hard/soft segment segregation in the longer time annealing experiments, changes in interdomain spacing were detected at the lowest annealing temperature (120 degrees C) used in this study. In combination with the findings from the synchrotron X-ray experiments, this demonstrates that domain reorganization is clearly, both time and temperature dependent. The results from X-ray scattering and AFM experiments are also supported by those from FTIR spectroscopy and thermal analysis.