The photodecomposition of acetone in presence and absence of hydrogen with light of wave‐length 2500A has been studied at temperatures from 70 to 300°C. Up to 160°C acetone alone yields negligible amounts of methane and ethane is the hydrocarbon predominantly produced. The proportion of methane increases rapidly with temperature, the activation energy of its formation in acetone being 8.6 kcal. In presence of hydrogen methane formation is negligible at 70°C, is marked at 160°C and increases rapidly with temperature with an activation energy of this increase equal to 6.4 kcal. At 300°C it represents 75 percent of the ethane‐methane product, which shows that photodissociation to methyl radicals is the major, if not exclusive primary process. Analysis of the data indicates that ethane formation is either a wall reaction with zero activation energy or a bimolecular association process of small activation energy. A decrease in ethane formation with increase of temperature is ascribed to a decrease in the stationary state concentration of methyl radicals. An activation energy of the reaction CH3+H2=CH4+H equal to 11±2 kcal. has been deduced. A mechanism to account for quantum yields less than unity in the low temperature range, consistent with primary dissociation into radicals, has been suggested.
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