Polyurethanes containing different soft and hard segments were investigated by fluorescence and scanning electron microscopy. The polarity dependence of the vibrational structure of the pyrene emission spectrum indicated the formation of aggregates at concentrations, which are significantly below the critical concentrations, which define the separation of dilute-semidilute domains. Unlike the samples with 4,4'-methylene diphenylene diisocyanate, the samples with 2,4-tolylene diisocyanate in hard segments give the fluorescence spectra in which the pyrene excimer appears. The supermolecular structures associated with the form of spherulites or of spherical micelles were detected by scanning electron n-Licroscopy. The results were compared with previous reports concerning viscometric data. (c) 2007 Wiley Periodicals, Inc.