In conventional electrochemical etching, contamination of the etched surface with electrolytes is unavoidable. Electrolyte-free electrochemical etching will be applicable to electronic device manufacturing and precision nanoscale processing as a low-cost and environmentally friendly process. We propose a new electrochemical etching method that requires no electrolytes, but rather ultrapure water. In principal, a metal surface can be etched by OH- ions. Thus, we prepared a sulfonated cathode that increases the OH- ion concentration and carried out electrolysis in ultrapure water and Cu etching. The resultant electrolysis current was 8.9-fold higher than that obtained using unmodified electrodes. Furthermore, nearly 100% etching efficiency was achieved in etching a Cu surface in combination with the sulfonated cathode through the anodic reaction Cu + 2 OH- -> Cu (OH)(2). The resultant etched surface had no pits; its roughness was 4.3 nm Ra. (c) 2006 The Electrochemical Society.