Transmission electron microscopy and the cathocloluminescence method have been used to study the transition region in 3C-SiC/6H-SiC heterostructures. It is shown that this region is. as a rule, constituted by alternating 3C-SiC and 6H-SiC layers, with possible inclusion of other silicon carbide polytypes. An assumption is made that this structure of the transition region can be explained in terms of the spinodal decomposition model. (C) 2014 Elsevier B.V. All rights reserved.