The photolysis of 2-methylcyclohexanone has been studied in the gas phase a t 313 nm, mainly at 100Β°C, over a range of pressures. The Hg(63P1) photosensitized decomposition has also been investigated at 100Β°C. Under conditions of high excitation and/or little collisional quenching the major products are carbon monoxide and the hydrocarbons: 1-hexene, trans-and cis-2-hexene and methylcyclopentane, with minor aldehyde formation. The various product ratios are presented in tabular form. At lower excitation energies, and with increased collisional deactivation, trans-and cis-5-heptenal become important products, and the translcis aldehyde ratio is seen to be slightly pressure dependent when all the systems are compared. Similarly, there is a small pressure dependence for the X hexenes/methylcyclopentane ratio. From experiments at 250Β°C the temperature dependence of this ratio was established, and for thermalized hexane-1,5diyl an activation energy difference (Ed -E,) = -1.3 kcal mo1-I has been determined for the disproportionation and combination of the biradical. The mechanism for the photolysis is discussed in terms of triplet state photochemistry and biradical intermediates as developed, in particular, by Frey and coworkers, this Journal, 16, 1337 (1984).