In situ electrochemical scanning tunneling microscopy measurements of the anodic oxidation of Cu(0 0 1) in 0.1 M NaOH are reported. Adsorption-induced surface reconstruction is observed in the underpotential range of oxidation with the formation of dimers of superimposed Cu atoms ejected from the substrate and stabilized by adsorbed OH groups, presumably in bridging positions. The reconstruction causes the reorientation of the substrate step edges and the formation of holes and ad-islands of monoatomic height. The dimers of superimposed Cu atoms are alternatively aligned along the <1 0 0> directions to form zig-zag arrangements. Long range ordering is observed in areas of limited lateral extension with c(2 x 6) and c(6 x 2) domains. In the potential range of Cu(I) oxide formation, a facetted Cu2O layer grows with a Cu2O(0 0 1)[1 1 0]parallel to Cu(0 0 1)[1 0 0] epitaxial relationship. The 45degrees rotation between the close-packed directions of the oxide lattice and metal lattice results from the orientation of the dimers of superimposed Cu atoms in the precursor adsorbed OH layer. The surface of the oxide layer is facetted due to a tilt of similar to 3% between oxide and metal lattices. Its (0 0 1) terraces have an identical chemical termination and are presumably hydroxylated. (C) 2003 Elsevier Science B.V. All rights reserved.