The photolysis of acetone – hydrogen chloride mixtures has been investigated at 150 °C. and at room temperature. A strong suppression of ethane formation with a corresponding large increase in the formation of methane results from additions of relatively very small amounts of hydrogen chloride to acetone. The importance of the reactionsandhas been demonstrated. The collision yield of reaction (1) at 28 °C. is 2 × 10−4, and, therefore, the upper limit for E1is 5.1 kcal. per mole. The effects observed at 150° and 28° indicate that, on the assumption of a zero activation energy and a steric factor of unity for combination of methyl radicals, E1 = 2.1 ± 1 kcal. per mole, andP1is approximately 7 × 10−3.