It　is　of　vital　importance　to　capture　137Cs+　and　90Sr2+　from　solutions　for　radionuclide　remediation　due　to　their　hazardous　nature　and　long　half-life.　Although　metal　organic　framework　(MOF)　materials　have　been　greatly　developed,　the　studies　of　their　application　in　the　capture　of　radioactive　cations　remain　rare.　Especially,　the　capture　mechanism　has　not　been　clarified.　Here,　we　present　a　new　In-MOF　material　with　the　potential　for　remediation　of　radioactive　Cs+　and　Sr2+　ions　by　ion　exchange,　namely,　[Me2NH2][In(TDC)2]·1.5DMA·1.5H2O　(FJSM-InMOF;　H2TDC　=　2,5-thiophenedicarboxylic　acid　and　DMA　=　N,N′-dimethylacetamide).　It　possesses　an　anionic　framework　of　[In(TDC)2]nn-　charge　balanced　by　dimethylammonium　cations.　It　could　be　easily　synthesized　via　a　one-pot　solvothermal　route.　Remarkably,　it　showed　excellent　β　and　α　radiation-resistances,　high　exchange　capacities　(qCsm　=　198.63　mg　g-1;　qSrm　=　43.83　mg　g-1),　impressive　selectivity　(KCsd　=　7.50　×　104　mL　g-1;　KSrd　=　9.49　×　105　mL　g-1),　high　ion-exchange　efficiency　(RSr　=　99.89%;　RCs　=　99.63%)　and　the　ability　of　convenient　elution　with　low　cost.　In　particular,　the　ion　exchange　mechanism　was　illuminated　by　an　unprecedented　observation　of　the　single-crystal　to　single-crystal　structural　transformation　in　the　Cs+-exchange　process.　This　work　paves　the　way　to　develop　highly　efficient　and　environmentally　friendly　MOFs　for　the　selective　capture　of　radionuclides.　©　2018　The　Royal　Society　of　Chemistry.