3-Diazo-2-butanone (1), 2,2,5,5-tetramethyl-4-diazo-3-hexanone (2), and 2-diazocyclohexanone (3) were photolyzed in the gas phase at low pressure. The course of the reactions was followed by FT IR spectroscopy. The Wolff rearrangement occurred to an extent of 70% for 1 and of about 40% for 2; the resulting ketenes were respectively somewhat photolabile and photostable. The ketene from 3 was quite unstable to photolysis, although it could be identified in the early part of the reaction. For irradiations at the laser line = 457.9 nm, the ratio of photolysis rates k¡/k2 was at least 300, which suggests a concerted mechanism for the rearrangement of 1. Photolysis of 1 in the presence of argon to increase intermolecular collisions raised the production of dimethylketene to over 95%. In contrast, the Wolff rearrangement of 2 occurred in the gas phase but not in solution. The gas-phase reaction path may have its origin in the excited vibrational states of an electronic isomer of 2 or in those of the corresponding car bene.-Diazo ketones, R1COC(N2)R2, are known to undergo the Wolff rearrangement to ketenes, R1R2C=C=0, under photolytic or thermolytic conditions. The ketenes are often not isolated but rather identified through their addition products such as esters. Studies of the rearrangement of -diazo ketones locked into specific conformations have suggested that these rearrangements are subject to conformational control.1 Thus, photolysis of 3,3,6,6-tetramethyl-2-diazocyclohexanone, which is locked into an s-Z conformation, in CC14 or dioxane solution produced the Wolff rearrangement product 2,2,5,5-tetramethylcyclopentanone in 96% yield, whereas photolysis of 2,2,5,5tetramethyl-4-diazo-3-hexanone, which is locked into an s-E conformation, in CC14, dioxane, or C12C=CC12 gave 2,2,4,5-tetramethyl-4-hexen-3-one in 78% yield and none of the Wolff rearrangement product di-tert-butylketene.1The photolysis of -diazo ketones most likely proceeds from the excited vibrational states of their singlet electronic isomers.2 From these states, competition can occur between photolysis and the collisional deexcitation of the molecules in the gas phase. The activation of these vibrational cascades by an increase in the frequency of intermolecular collisions may change the relative proportions