Many aspects of the electrochemistry of copper in aqueous acid solution are dominated by the presence of low coverage, high energy, metastable surface states which constitute surface active sites. Such states undergo quasireversible redox transitions within the double-layer region, and these species play a vital role in electrocatalytic and electrodeposition processes in the case of this electrode system. In the present work this aspect of copper electrochemistry is explored using alternating current impedance techniques and one of the main conclusions drawn is that there are often important aspects of surface electrochemistry that can only be rationalized in terms of the unusual properties of the active, highly defective, state of the surface. (c) 2007 The Electrochemical Society.