The products CO and CO2 are observed experimentally for the first time in the HCCO + O2 reaction. Bothproduct molecules have identical appearance times and are formed simultaneously rather than sequentially.Both CO and CO2 are produced with significant internal excitation. The product channel producing H + CO+ CO2 accounts for at least 90% of the reaction products at 298 K. The results explain the observation of“prompt” CO2 in acetylene oxidation chemistry.