The Cl atom-initiated oxidation of CH3Cl was studied over the temperature range 264-336 K. The observed products, measured using Fourier transform infrared (FTIR) spectroscopy, were HC(O)Cl, CO, and HCl. At a given temperature and total pressure, the product ratio HC(O)Cl/CO increased linearly with O-2 partial pressure, reflecting a competition for available CH2ClO radicals between decomposition and reaction with O-2 : CH2ClO + M --> HCO + HCl + M(1); CH2ClO + O-2 --> HC(O)Cl + HO2 (2). At 700 Torr total pressure the observed rate constant ratio k(2)/k(1) was fit by the expression 5.6(-4.9)(+33) X 10(-23) exp(3300 +/- 600/T) cm(3) molecule(-1). Assuming an activation energy of 1.5 +/- 0.5 kcal mol(-1) for reaction 2, an activation energy of 8.6 +/- 1.9 kcal mol(-1) is estimated for the intramolecular elimination of HCl via reaction 1.