The　characteristics　of　CO　and　NO　molecules　at　Cu2+　and　Cr3+　ion　sites　on　the　CuCr2O4　(100)　surface　have　been　studied　by　first　principles　calculations　based　on　spin-polarized　density　functional　theory　(DFT).　The　calculated　results　show　that　adsorption　energies　for　X-down(C,　N)　adsorption　vary　in　the　order:　Cu2+-CO＞Cr3+-　NO　approximate　to　Cr3+-CO＞Cu2+-NO.　CO　molecules　are　preferentially　adsorbed　at　Cu　sites,　whereas　NO　molecules　adsorb　favorably　at　Cu2+　and　Cr3+　ion　sites.　The　C-O　and　N-O　stretching　frequencies　are　red-shifted　upon　adsorption.　Combining　the　analysis　of　frontier　molecular　orbitals　and　Mulliken　charge,　for　CO　and　NO　X-clown　adsorption　systems,　the　5　sigma　orbitals　donate　electrons　and　the　2　pi*　orbitals　obtain　back-donated　electrons.　Although　for　NO　with　O-down　adsorption　systems,　the　NO-2　pi*　orbitals　obtain　back-donated　electrons　from　substrates　without　5　sigma-donation.　Coadsorption　calculations　show　the　CO/NO　mixture　adsorb　selectively　at　the　Cu2+　ion　site　but　simultaneously　at　the　Cr3+　ion　site,　respectively.　(C)　2008　Wiley　Periodicals,　Inc.