The excess energy dependence of S 1 → S 0 internal conversion in gas phase aliphatic ketones is examined through a quasi-diatomic molecule model calculation which treats the optically excited carbonyl stretching vibration as the only important accepting mode. It is shown that the sharp decrease in the fluorescence lifetime with increasing excitation energy in certain halogenated acetones can be rationalized in terms of a rapidly increasing rate of internal conversion as the vibrational energy content of S 1 is increased. The consequences of the results for the interpretation of photochemistry in these compounds are briefly discussed.