Catalytic　generation　of　reactive　oxygen　species　has　been　developed　as　a　promising　methodology　for　tumor　therapy.　Direct　O2•-　production　from　intratumor　oxygen　exhibits　exceptional　tumor　therapeutic　efficacy.　Herein,　this　therapy　strategy　is　demonstrated　by　a　pH-responsive　hybrid　of　porous　CeO2　nanorods　and　sodium　polystyrene　sulfonate　that　delivers　high　oxidative　activity　for　O2•-　generation　within　acidic　tumor　microenvironments　for　chemodynamic　therapy　and　only　limited　oxidative　activity　in　neutral　media　to　limit　damage　to　healthy　organs.　The　hydrated　polymer-nanorod　hybrids　with　large　hydrodynamic　diameters　form　nanoreactors　that　locally　trap　oxygen　and　biological　substrates　inside　and　improve　the　charge　transfer　between　the　catalysts　and　substrates　in　the　tumor　microenvironment,　leading　to　enhanced　catalytic　O2•-　production　and　consequent　oxidation.　Together　with　successful　in　vitro　and　in　vivo　experiments,　these　data　show　that　the　use　of　hybrids　provides　a　compelling　opportunity　for　the　delivery　selective　chemodynamic　tumor　therapy.