AbstractDecabromodiphenyl ether (BDE-209), the major congener in the high volume industrial flame retardant mixture “DecaBDE”, has become a ubiquitous environmental con- taminant. In the present work, combined experimental and theoretical studies have been underta- ken on the structure and vibrational spectra of BDE-209. The FT-IR (400–4000 cm-1) and FT-Raman spectra (100–4000 cm-1) of BDE-209 were recorded, while density functional B3LYP calculations were employed in conjunction with the 6-31G(d) basis set for investigating the corresponding geometric structure and vibrational spectroscopic properties. Besides, the detailed interpretations of fundamental vibrations were performed on the basis of experimental results and potential energy distribution (PED) of the vibrational modes. Optimized structures of the title compound were interpreted and compared with the earlier reported experimental values, which yield good agreement. Finally, the measured and calculated harmonic vibrational wavenumbers were compared with each other, and they were found to be in good accordance.