We present an electrochemical study of Au3Cu (0 0 1) single crystal surfaces in 0.1 mol dm(-3) H2SO4 and 0.1 mol dm(-3) H2SO4 + 0.1 mmol dm(-3) HCl, and of Cu3Au (0 0 1) in 0.1 mol dm(-3) H2SO4. The focus is on in situ scanning tunneling microscopy experiments. The changes of the surface morphology, which are time-and potential-dependent, have been observed, clearly resolving single atomic steps and mono-atomic islands and pits. Chloride additives enhance the surface diffusion and respective morphologies are observed earlier. All surfaces have shown considerable roughening already in the passive region far below the critical potential. (C) 2010 Elsevier Ltd. All rights reserved.