The geometries, energies, and vibrational frequencies of the reactants, transition structures, intermediates, and products of the reaction of the acetyl radical with atomic chlorine have been determined by ab initio molecular orbital theory at the second-order Moller Plesset perturbation (MP2) level. Energies have been recalculated at the quadratic configuration interaction QCISD(T) level by using geometries obtained at MP2 level. The energy of the initial acetyl chloride adduct CH3COCl (1), formed by barrier-free combination, lies 78 kcal/mol below the reactants. Two major reaction routes are open to the chemically activated adduct 1 : molecular dissociation to H2C=C=O + HCl (3), and the secondary formation of ketene via 1-chlorovinyl alcohol (2). Both these processes are energetically feasible to the thermal reactants and should hence lead to a spontaneous emission of a vibrationally hot HCl molecule as observed by Maricq et al. (Int. J. Chem. Kinet. 1997, 29, 421). The thermodynamically most stable products, CH3Cl + CO, should preferably be formed via direct displacement of CO from CH3CO by Cl; this reaction proceeds via a loose complex between Cldelta- and CH3COdelta+, which explains the delayed emission of CO in the diode laser study of the Cl + CH3CO reaction. The energy barrier for decarbonylation of the adduct 1 is quite high and thereby is not accessible to the thermal reactants. The present potential energy surface reveals this reaction to be a capture-limited association-elimination reaction with a very high and pressure-independent rate coefficient.