The interaction of CO with differently modified polycrystalline ZnO has been studied by FTIR spectroscopy under ultrahigh vacuum conditions (UHV-FTIRS). After exposing the clean, adsorbate-free ZnO nanoparticles to CO at 110K we observe an intense vibrational band at 2187 cm(-1) which is assigned to a majority of CO species bound to the Zn2+ sites on the mixed-terminated ZnO(1 0 (1) over bar 0) surface. After the exposure of CO2-pretreated ZnO nanoparticles to CO at 110K, a new CO band is observed at 2215 cm(-1), which originates from CO species adsorbed on the "free" Zn sites embedded within the (2 x 1) tridentate carbonate structure on the ZnO(1 0 (1) over bar 0) surface. UHV-FTIRS data recorded at different sample temperatures demonstrate that the binding energy of CO on polycrystalline ZnO is substantially increased in the presence of pre-adsorbed CO2. The presence of hydroxyl species on the ZnO powder particles does not lead to substantial changes of the CO vibrational bands detected at 110K under UHV conditions. (C) 2010 Elsevier B.V. All rights reserved.