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The growing demand for luminescent nanomaterials intended for various applications increases the necessity to develop and improve approaches to the creation of highly efficient nanosized phosphors. In current study, an approach to enhancing the efficiency of red luminescence of monoclinic Y2O3:Eu3+with the particle size of ∼20 nm by creating the oxygen-enriched environment during the gas-phase synthesis was developed. To investigate the effect of oxygen amount during the synthesis on characteristics of the phosphor, a series of nanostructured Y2O3:Eu3+with the addition of 0–40 vol% O2to the main buffer gas Ar was synthesized by laser vaporization. It is shown that the amount of added O2exerts virtually no effect on the phase composition and particle size, but significantly improves the luminescent characteristics of Y2O3:Eu3+. Thus, the addition of 30 vol% O2leads to virtually a 20-fold growth in the photoluminescence (PL) intensity caused by 5D0→7F0–4transitions in Eu3+and an increase in the absolute PL quantum yield from 3% to 53% (λex = 395 nm) compared to the sample synthesized without O2. The addition of oxygen also improves the emission color coordinates from (0.571, 0.320) to (0.630, 0.322) due to the removal of a considerable fraction of oxygen vacancies. The synthesized nanopowders are shown to be highly stable: upon storage under ambient conditions for two years. Quantum yield (QY) of the samples decreases by less than 2%. It is expected that the key features underlying the proposed approach will be useful for various methods used to synthesize oxide nanophosphors.