A high-purity electrolytic copper single crystal with (1 1 0) [0 0 1] initial orientation was uniformly rolled and then, additionally, channel die compressed after switching the rolling direction with the transverse direction. In spite of room temperature deformation, deformation twins and other unexpected structural elements were formed in the microstructure. These elements were identified as copper polytype of the 9R type. It is believed that the mechanism of formation of such features is a martensitic one rather than traditional dislocation pole mechanism. (C) 2003 Elsevier Science B.V. All rights reserved.