The　development　of　core-shell　nanoclusters　with　controllable　composition　is　of　utmost　importance　as　the　material　properties　depend　on　their　constituent　elements.　However,　precisely　tuning　their　compositions　at　the　atomic　scale　is　not　easily　achieved　because　of　the　difficulty　of　using　limited　macroscopic　synthetic　methods　for　atomic-level　modulation.　In　this　work,　we　report　an　interesting　example　of　precisely　regulating　the　core　composition　of　an　inorganic　core-shell-type　cobalt　polyoxoniobate　[Co26Nb36O140]32−　by　controlling　reaction　conditions,　in　which　the　inner　Co-core　composition　could　be　tune　while　retaining　the　outer　Nb-shell　composition　of　resulting　product,　leading　to　a　series　of　isostructural　species　with　a　general　formula　of　{Co26-nNb36+nO140}　(n　=　0–2).　These　rare　species　not　only　can　display　good　powder　and　single-crystal　proton　conductivities,　but　also　might　provide　helpful　and　atomic-level　insights　into　the　syntheses,　structures　and　composition　modifications　of　inorganic　amorphous　core-shell　heterometal　oxide　nanoparticles.　©　2023