Based　on　the　strategy　of　increasing　the　number　of　oxygen　vacancies　to　improve　the　catalytic　performance,　we　have　developed　a　novel　electrochemical　sensor　based　on　the　multivalent　metal　oxides　cerium　dioxide　and　manganous　oxide　(Mn3O4/CeO2)　for　reliable　determination　of　extracellular　hydrogen　peroxide　(H2O2)　released　from　living　cells.　The　Mn3O4/CeO2　nanocomposite　was　characterized　by　high-resolution　transmission　electron　microscopy,　energy-dispersive　X-ray　spectroscopy,　X-ray　diffraction,　and　X-ray　photoelectron　spectroscopy.　The　electrochemical　performance　of　the　Mn3O4/CeO2　nanocomposite　modified　glassy　carbon　electrode　(Mn3O4/CeO2/GCE)　was　investigated.　Owing　to　the　abundant　oxygen　vacancies　and　strong　synergistic　effect　between　the　multivalent　Ce　and　Mn,　the　sensor　exhibited　excellent　catalytic　activity　and　selectivity　for　the　electrochemical　detection　of　H2O2　with　a　low　quantitation　limit　of　2　nM.　Moreover,　Mn3O4/CeO2/GCE　exhibited　excellent　reproducibility,　repeatability,　and　long-term　storage　stability.　Because　of　these　remarkable　analytical　advantages,　the　constructed　sensor　was　able　to　determine　H2O2　released　from　living　cells　with　satisfactory　results.　The　results　showed　that　the　Mn3O4/CeO2　sensor　is　a　promising　candidate　for　a　nanoenzymatic　H2O2　sensor　with　the　possibility　of　applications　in　physiology　and　diagnosis.