Periodic　density　functional　theory　calculations　have　been　performed　to　study　the　electronic　properties　of　bimetallic　oxide　CrW2O9　clusters　adsorbed　on　MgO/Ag(001)　ultrathin　films　(＜1　nm).　Our　results　show　that　after　deposition　completely　different　structures,　electronic　properties　and　chemical　reactivity　of　dispersed　CrW2O9　clusters　on　ultrathin　films　are　observed　compared　with　that　on　the　thick　Mg0　surface.　On　the　thick　Mg0(001)　surface,　adsorbed　CrW2O9　clusters　are　distorted　significantly　and　just　a　little　electron　transfer　occurs　from　oxide　surface　to　clusters,　which　originates　from　the　formation　of　adsorption　dative　bonds　at　interface.　Whereas　on　the　MgO/Ag(001)　ultrathin　films,　the　resulting　CrW2O9　clusters　keep　the　cyclic　structures　and　the　geometries　are　similar　to　that　of　gas-phase　[CrW2O9](-).　Interestingly,　we　predicted　the　occurrence　of　a　net　transfer　of　one　electron　by　direct　electron　tunneling　from　the　MgO/Ag(001)　films　to　CrW2O9　clusters　through　the　thin　MgO　dielectric　barrier.　Furthermore,　our　work　reveals　a　progressive　Lewis　acid　site　where　spin　density　preferentially　localizes　around　the　Cr　atom　not　the　W　atoms　for　CrW2O9/MgO/Ag(001)　system,　indicating　a　potentially　good　bimetallic　oxide　for　better　catalytic　activities　with　respect　to　that　of　pure　W3O9　clusters.　As　a　consequence,　present　results　reveal　that　the　adsorption　of　bimetallic　oxide　CrW2O9　clusters　on　the　MgO/Ag(001)　ultrathin　films　provide　a　new　perspective　to　tune　and　modify　the　properties　and　chemical　reactivity　of　bimetallic　oxide　adsorbates　as　a　function　of　the　thickness　of　the　oxide　films.　(C)　2016　Published　by　Elsevier　B.V.