High-quality AlN and GaN bulk crystals were etched in a KOH aqueous solution or a KOH/H2O2 mixture. As etched surfaces were characterized by scanning electron microscope (SEM), atomic force microscope (AFM), and X-ray photoelectron spectroscopy (XPS). It was found that the N-polar surfaces had approximately 900 and 20 times higher etch rate than the III-polar surface for AlN and GaN, respectively. AlN had a higher total etch rate than GaN at the same condition. Hexagonal hillocks were observed on N-polar face with {10-1-1} boundary planes for both AlN and GaN, while metal polar surfaces remAlNed smooth. Formation of aluminum oxide/hydroxide AlOx(OH)(y) was confirmed by XPS on as etched N polar AlN surface; the addition of H2O2 resulted in a higher total surface oxygen concentration. The smoothening effect by adding H2O2 oxidizer was explAlNed by the formation and dissolution of aluminum oxide/hydroxide. (C) 2013 Elsevier B.V. All rights reserved.