Photoisomerization channels in the vapor phase photochemistry of cyclohexanone (CH) are studied by performing photolysis with 311 nm UV light, and analyzing the photoproducts by means of FTIR spectroscopy. In addition to 5-hexenal, identified previously as a photoisomerization product, we observe an efficient parallel isomerization channel producing butylketene as a primary photoproduct, but it remained unidentified in earlier studies. Calculation at DFT/B3LYP/6-311++G** level predicts formation of this ketene to be favorable both with respect to thermodynamic and kinetic factors. (C) 2017 Elsevier B.V. All rights reserved.