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The photolysis of acetone has been investigated in the presence of the vapour of iodine labelled with radioactive 1311, using the output of a high-pressure mercury arc filtered so as to give only radiation in the neighbourhood of 3 130 A. The iodine quenches almost completely the secondary reactions of the radicals produced in the photolysis, but appears also to have a specific deactivating effect on one of the activated molecules involved in the photolysis, with the result that the quantum yield of methyl iodide depends on the pressures of both acetone and iodine. A mechanism is discussed which explains the observed results.* A brief account of the technique employed has been given previous1y.s 7 We are grateful to the Atomic Energy Research Establishment for co-operation in the supply of neutron-irradiated material.

The ultra-violet photolysis of methyl ethyl ketone + iodine mixtures has been studied using radioactively labelled iodine to facilitate the separation and estimation of the products. Information has been obtained about the primary quantum yield of free methyl and ethyl radicals at two wavelengths. It is shown that the two-stage type of mechanism which appears to operate in acetone + iodine mixtures is involved with methyl ethyl ketone also, although it is of much less significance.

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