The adsorption of NCO over two types of surface defects on silica surface, equivalent toSi* and equivalent toSi-O*, were studied using the density functional theory formalism. The adsorption is preferred on equivalent toSi* site in agreement with experimental results. The NCO adsorption is also studied over atomic and dimer copper deposited on silica defects and, for comparison reasons, on free Cu and Cu-2. While in the first case the support does not affect the Cu-NCO bonding, it enhances that interaction in the second case. This behaviour is related to the higher charge transfer from the dimer to NCO, which produces a strong Cu-NCO bonding which has an important ionic character. Besides, the adsorbate modifies the metal-support interaction due to an electrostatic bond enhancement between the terminal Cu atom and a regular bridging O atom of the silica surface. The NCO asymmetric stretching frequencies were also calculated and analysed. (C) 2004 Elsevier B.V. All rights reserved.