The photo-induced formation of peroxide ions on the surface of cubic Ln₂O₃ (Ln = Nd, Sm, Gd) was studied by in situ microprobe Raman spectroscopy using a 325 nm laser as excitation source. It was found that the Raman bands of peroxide ions at 833–843 cm⁻¹ began to grow at the expense of the Ln³⁺–O²⁻ bands at 333–359 cm⁻¹ when the Ln₂O₃ samples under O₂ were continuously irradiated with a focused 325 nm laser beam at temperatures between 25–150 °C. The intensity of the peroxide Raman band was found to increase with increasing O₂ partial pressure, whereas no peroxide band was detected on the Ln₂O₃ under N₂ as well as on the samples first irradiated with laser under Ar or N₂ followed by exposure to O₂ in the dark. The experiments using ¹⁸O as a tracer further confirmed that the peroxide ions are generated by a photo-induced reaction between O₂ and the lattice oxygen (O²⁻) species in Ln₂O₃. Under the excitation of 325 nm UV light, the transformation of O₂ to peroxide ions on the surface of the above lanthanide sesquioxides can even take place at room temperature. Basicity of the lattice oxygen species on Ln₂O₃ also has an impact on the peroxide formation. Higher temperature or laser irradiation power is required to initiate the reaction between O₂ and O²⁻ species of weaker basicity.