Infrared chemiluminescence has been observed from the reaction of atomic oxygen with acetylene under a variety of conditions. The main emission is identified as arising from vibrationally excited carbon monoxide in its fundamental, and first and second overtone regions. The vibrational energy distribution is approximately Boltzmann up to about the 13th vibrational level but with a slight overpopulation in the low vibrational levels. Above v= 13 there is a progressively more pronounced underpopulation. A low rotational temperature is observed. It is conculed that the data are consistent with the carbon monoxide being formed primarily as a result of the reactions O + C2H2→ CO + CH2+ 51 kcal./mole (i), O + CH2→ CO + 2H + 71 kcal./mole (ii)(i) and (ii). The effects of pressure, acetylene partial pressure, and molecular oxygen and nitrogen are discussed in terms of this mechanism.