Copper underpotential deposition (Cu upd) studies have been carried out on flat and stepped Au single crystal surfaces to demonstrate the influence of crystal orientation on Cu upd characteristics. Seven different Au single crystal surfaces were employed, Au(111), Au(110), Au(100), Au(554), Au(775), Au(332) and Au(755). Voltammetric contributions for Cu upd at both (110) and (100) oriented steps were similar to those on the extended (110) and (100) surfaces. In contrast, the formation of the (root3 x root3) R30degrees structure on (111) terraces is strongly dependent on their width. An increase in step density and the corresponding reduction in (111) terrace width favour the (root3 x root3) R30degrees --> (1 x 1) phase transition. This could result from the influence of structural or electronic (Smoluchowski) effects on the energetics and/or kinetics of the phase transition. (C) 2004 Elsevier B.V. All rights reserved.