First-principles　calculations　based　on　density　functional　theory　(DFT)　and　the　generalized　gradient　approximation　(GGA)　have　been　used　to　study　the　adsorption　of　CO　and　NO　molecules　on　the　Cu　2　O(1　1　1)　surface　in　the　presence　of　oxygen　vacancy.　The　calculations　employ　slab　geometry　and　periodic　boundary　conditions　with　partial　relaxation　of　atom　positions.　Two　molecular　orientations,　X-　and　O-down　(X　=　C,　N),　at　two　distinct　sites,　Cu　1C　and　oxygen　vacancy　sites,　have　been　considered.　Total　energy　calculations　indicate　that　the　Cu　1C　position　is　relatively　more　favored　than　the　oxygen　vacancy　site.　The　predicted　binding　energies　are　144.5　kJ　mol　-1　(CO)　and　124.1　kJ　mol　-1　(NO),　respectively.　The　C-O　and　N-O　stretching　frequencies　are　unequally　red-shifted　upon　adsorption.　Upon　adsorption　at　Cu　1C　site,　CO　molecule　was　found　to　bind　to　Cu　1C　atoms　in　vertical　configuration　whereas　NO　molecule　adsorption　in　tilted　mode.　While　upon　adsorption　at　oxygen　vacancy　site,　CO　and　NO　molecules　are　both　vertical　to　the　Cu　2　O(1　1　1)　surface.　Interestingly,　we　found　that　their　adsorption　properties　on　oxygen　vacancy　site　are　dependent　on　the　defect　density.　As　the　density　of　defective　sites　increased,　the　adsorption　energies　of　the　defect-XO　configuration　increase　and　the　N-O　bond　is　continuously　weakened　whereas　the　C-O　bond　remains　constant.　Therefore,　such　a　process　favors　the　dissociation　of　the　NO　molecule　and　has　a　small　influence　on　the　adsorbed　CO　molecule.　©　2008　Elsevier　B.V.　All　rights　reserved.