Hydrogen peroxide (HP) plays an indispensablerole in living organisms,being both an intracellular messenger and a substrate or byproductof a number of enzymes. In this way, HP sensing is important for monitoringthe activity of various cell systems and assessing the redox balanceof the cell as whole. HP sensors based on luminescent inorganic nanoparticlescan be considered as a prospective alternative to traditional dye-and enzyme-based sensors, which usually are unstable and nonreversible.The undoped (CeO2-x ) and Eu3+-doped (CeO2-x :Eu3+) colloidal ceria nanoparticles studied in the paper provideHP detection by reversible quenching of Eu3+ (590 nm) andCe(3+) (430 nm) luminescence bands. The dynamics of Eu3+ and Ce3+ luminescence quenching and recoveryduring HP-nanoceria interaction provides an insight into the microscopicmechanisms of HP sensing by CeO2-x and CeO2-x :Eu3+ nanoparticles.Both CeO2-x and CeO2-x :Eu3+ luminescent sensors are reversibleand their recovery rates can be sufficiently increased by temperatureand continuous UV irradiation. At the same time, Eu3+ ionsdeteriorate the catalase-mimetic activity of CeO2-x NPs and worsen their antioxidant properties, whichshould be kept in mind while using these sensors in biological media.