The etch-pit density method was employed to investigate ZnO substrates on both polar faces. A common wet-chemical etching approach was used to reveal etch pits on a zinc-polar face. An alternative thermal-treatment procedure was developed in order to reveal etch pits on an oxygen-polar face, since wet chemical etching is not applicable for etch-pit density investigation of this face. The dependence of thermal treatment on temperature and gas environment is also described. (C) 2007 The Electrochemical Society.