The thermal decomposition of methylene chloride has been studied in the temperature range 500 to 650 °C by both the static technique of pressure-time measurement and the use of a flow system in conjunction with gas chromatographic analysis. The reaction, which leads principally to carbon and hydrogen chloride is characterized by a slow acceleration, the rate of which decreases with the vessel diameter. In vessels of diameter less than 5 mm the reaction is almost completely inhibited. The reaction rate is increased by the addition of inert gas, nitric oxide and, particularly, by dichlorethylene. Using the flow system a number of chlorinated hydrocarbons were detected as minor products of the reaction and their rate of formation relative to the major products was followed in detail. By identifying some of these as radical recombination products and one, dichlorethylene, as a degenerate branching agent, a delayed branching mechanism has been deduced which explains most of the kinetic features of the reaction as well as the formation of the observed minor products. This involves the production of the intermediate, dichlorethylene, in a chain carried by chlorine atoms and dichlormethyl radicals, and the conversion of this to carbon and hydrogen chloride by a coupled chain also involving chlorine atoms. The average primary chain length has been estimated as fifteen by measurement of the rate of formation of the supposed recombination products, but this figure is uncertain since the termination products appear to be destroyed in turn by chlorine atoms generated in the main chain.