We report on optimized wet chemical processes for both the O-polar and Zn-polar faces of wurtzite bulk ZnO single crystals. Different solutions were tested to achieve controllable etching. For the O-polar ZnO surface, a controlled etch rate of 3.8 mu m/min was observed using an acid mixture of H3PO4/CH3COOH/H2O as an etchant. Fine-patterning of the O-polar surface with moderate etch rates and high reproducibility can be obtained using an aqueous 5% NH4Cl solution. In comparison, the Zn-polar ZnO surface etches significantly slower in HCl solution and exhibits strong pH dependence. Nevertheless, pH control enables reproducible etching even of the Zn-polar surface. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3519999] All rights reserved.