The products CO and CO2 are observed experimentally for the first time in the HCCO + O-2 reaction. Both product molecules have identical appearance times and are formed simultaneously rather than sequentially. Both CO and Co-2 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.