Tip-induced metal deposition was employed to fabricate nanometer sized Cu clusters on Au3Cu(001) electrodes using an electrochemical scanning tunneling microscope (EC-STM). Due to the mixed Au and Cu termination of the substrate surface, an underpotential deposition (UPD) layer of copper covers only Au sites. Investigation of the anodic oxidation and dissolution of the Cu clusters give evidence for a strong inhibition of surface diffusion reactions due to this partial UPD layer. Based on a comparison of the nanocluster dissolution behavior with dealloying reactions of the substrate, it is concluded, that the nanostructures consist of a mixture between copper and gold due to interfacial alloying effects. This offers a novel explanation for the unexpected and unusually high stability of the clusters frequently reported in literature. (C) 2003 The Electrochemical Society.