The adsorption of hydroxyl and calcium hydroxyl ions (OH- and CaOH+) on pyrite (1 00) surfaces was studied using first-principles calculations to investigate the depression of pyrite by NaOH and CaO. The calculation results showed that the adsorption of CaOH+ on pyrite surfaces was stronger than the adsorption of OH-. The surface Fe atoms were the active sites for the two adsorbates, which is not conducive to the interaction of xanthate with the pyrite surface. In addition, partial surface S atoms were covered by Ca in CaOH+, which would not be favourable for the copper activation of pyrite. The OH- obtained electrons from the surface, whereas CaOH+ lost electrons to the surface. The loss of electrons resulted in the accumulation of electrons on the surface, which hindered the oxidation of pyrite and the formation of dixanthogen. Flotation tests showed that the Cu activation of pyrite after depression by CaO was more difficult than after depression by NaOH. This result was consistent with the calculated result that partial surface S sites, which were the active sites of Cu adsorption, were covered by Ca; this coverage resulted in the Cu activation becoming difficult. (C) 2011 Elsevier Ltd. All rights reserved.