First principles investigation based on density functional theory is done to study the adsorption of CO and NO on polar ZnO(0002) surface. Adsorption systems consist of clean and defective ZnO surfaces, taking into account the existence of pre-adsorbed species such as OH. It is found that the clean ZnO is selective toward NO adsorption. However, the inclusion of vacancy removes the selectivity on NO in favor of CO adsorption. The availability of OH on surface prior to adsorption improves the overall adsorption on defective surfaces and modifies the trend of selectivity. (C) 2017 Elsevier B.V. All rights reserved.