Photoexcitation of glyoxal at wavelengths over the range of 395-414 nm was observed to initiate a chemical reaction that produces the HCO radical in addition to the photolytic production of HCO. The technique of dye laser flash photolysis coupled to cavity ring-down spectroscopy was used to determine the time dependence of the HCO radical signal, analysis of which suggests that the chemical source of HCO is the self-reaction of triplet glyoxal (HCO)(2)(T-1) + (HCO)(2)(T-1) -> 2HCO + (HCO)(2) As the photoexcitation wavelength increases, the production from the triplet glyoxal reaction increases relative to that of HCO from direct photolysis, and at 414 nm, the dominant source of HCO in the system is from the self-reaction of the triplet. The formation of HCO via this process complicates the assignment of the photolysis quantum yield at longer wavelengths and may have been overlooked in some previous glyoxal photolysis studies.