Glyoxal and other dicarbonyl compounds have been detected in tropospheric air in rural areas and polluted urban areas. Accurate absorption cross-sections for the near UV spectrum of glyoxal are important since the value of this chemical characteristic is necessary for calculating effective quantum yields and ultimately the rates and products of photolysis. In this study, comprehensive measurements were made for the 2 80 nm gas-phase absorption spectrum of glyoxal. In addition, molecular orbital calculations were carried out and the results used as a guide to identify the vibration responsible for the weak structure features of the continuous area of the glyoxal spectrum. That vibration was found to be an ag CO stretch vibration mode of the glyoxal molecule.