AbstractThe infrared and Raman spectra of diacetyl and diacetyl‐d6 have been measured, and vibrational assignments are proposed for the fundamental bands and for several infrared active combination bands. The Raman spectrum of diacetyl‐d6 has not hitherto been determined, and only a few of the stronger Raman bands of diacetyl have been reported in previous work.For both molecules a planar trans structure with C2h symmetry fits the experimental data; this confirms the conclusions of earlier investigators based on the analysis of the infrared and fluorescence spectra and on electron diffraction measurements.