We investigated vulcanized rubber structures by energy-filtering transmission electron microscopy (EFTEM) and by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Both techniques revealed that numerous particles with the diameter of about 20 nm, which could not be observed by conventional TEM, were distributed in the rubber matrix. Further examination based on electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray (EDX) analysis indicated that those particles are composed of ZnS clusters with the sizes of approximately 3-5 nm, which are produced as a by-product in the vulcanization. We believe that the formation of ZnS clusters in a rubber network is one of the origins of rubber heterogeneities. (c) 2007 Elsevier Ltd. All rights reserved.