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　(2O9　clusters　on　ultrathin　films　are　observed　compared　with　that　on　the　thick　MgO　surface.　On　the　thick　MgO(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.　©　2016,　Elsevier　B.V.　All　rights　reserved.