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The optical properties of Yb 3+ and Ho 3+ co-doped Y 2 BaZnO 5 , synthesized by solid-state reactions, are investigated in detail. Under 977 nm excitation (~2.5 W/cm 2 ), bright green upconversion emission is observed. Concentration dependence studies at room temperature show that relatively high infrared to visible upconversion efficiencies are obtained with values up to ~2.6%. The results of power dependence studies and temperature dependent lifetime measurements allow us to determine the dominant upconversion mechanisms in Yb 3+ : Ho 3+ co-doped Y 2 BaZnO 5 oxides. The materials presented in this paper constitute new and efficient upconversion phosphors which may find utility in a variety of applications.