We report on the study of the electrochemical behavior of copper and brass electrodes in concentrated aqueous KOH solutions. Passive films are formed on the surface of anodically polarized, pure polycrystalline copper and a Cu-30Zn alloy (alpha-brass), in aqueous 30% (7 M) and 45% (11.7 M) KOH electrolyte solutions, at two characteristic potentials (corresponding to the two major anodic peaks in the voltammograms of these systems). The structure and composition of the films were determined by ex situ X-ray diffraction (XRD), in situ and ex situ micro-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The formation of crystalline cubic Cu2O, on the surface of Cu and Cu-30Zn electrodes in highly concentrated KOH solutions, occurs at potentials above -400 mV vs a standard hydrogen electrode (SHE). A porous passive layer is formed at potentials above -150 mV vs SHE on pure Cu surfaces. In a 30% KOH solution, this surface layer consists of Cu(OH)(2), while in a 45% KOH solution, the surface films comprise a Cu2O/Cu(OH)(2) structure with the Cu(2)Oas the inner layer. The anodic polarization of Cu-30Zn (alpha-brass) leads to a complete dezincification of the surface, at potentials >-350 mV vs SHE, in both electrolyte solutions. Porous surface layers are formed on brass at these anodic potentials and have duplex structure, Cu2O/Cu(OH)(2), where the Cu(OH)(2) content increases with an increase in the KOH concentration. At potentials >-150 mV vs SHE, the Cu(OH)(2) layer shows a preferential orientation of (020). (c) 2006 The Electrochemical Society.