The Raman features of neptunium dioxide (NpO2) and uranium–neptunium mixed oxides were investigated. Raman spectra of 16O and 17O-NpO2 enriched sample were measured with excitation energy range of Ei = 1.91–2.54 eV. Raman spectra of both NpO2 samples and the uranium–neptunium rich mixed oxides show an asymmetry at low energies of the Γ25′ (T2g) mode. The asymmetry was evidenced to originate from an electronic band at 431 cm–1 which originates from crystal field splitting. This assignment is in good agreement with theoretical models for NpO2 with the hypothesis that the electron–phonon bound state is decoupled at high temperatures. The asymmetry of the Γ25′ mode can be considered as a fingerprint of the NpO2 lattice. ; JRC.G.I.3-Nuclear Fuel Safety