The photolysis of trans-2,3-dimethylcyclopentanone has been investigated in the gas phase mainly at 3130 Å and 100°C. The major products of the decomposition are cis-2,3-dimethylcyclopentanone, cis- and trans-4-methylhex-4-enal, 2,3-dimethylpent-4-enal, cis- and trans-1,2-dimethylcyclobutane, cis- and trans-but-2-ene, ethylene, and carbon monoxide. These products are consistent with the formation of biradical intermediates. The variations of the product ratios have been measured as a function of pressure, temperature, wavelength, and added gases. Quantum yields for the various modes of decomposition have been determined over the pressure range studied. Benzene-photosensitised decompositions and liquid-phase photolyses in presence and absence of penta-1,3-diene, combined with a reinterpretation of previous data for cyclopentanone, favour the intermediacy of the triplet state in the decomposition of cyclopentanones.