Cyclopentanone vapor, when excited by light of wavelength 3130 Å, gives rise to four different photoproducts and fluorescence. The photochemistry of the molecule has been investigated by Srinivasan (J. Am. Chem. Soc.81, 1546 (1959); 83, 4344, 4348 (1961)). In the present work the fluorescence spectrum and fluorescence efficiency of cyclopentanone vapor are reported for the first time. The fluorescence spectrum is identical with that obtained from liquid solutions of cyclopentanone, with a maximum at 4100 Å. Time, temperature, oxygen, and inert gases do not affect the spectrum, which contains no component ascribable to phosphorescence, maximum 4450 Å. Beer's law is obeyed.The fluorescence efficiency is also found to be independent of the variables temperature and pressure of inert gas, cyclopentanone, and oxygen. This is in marked contrast with the behavior of the photochemical yields (see work by Srinivasan). The yields of cyclobutane and ethylene tend to increase at high vibrational energy content of the excited molecule (low pressure, high temperature, short exciting wavelengths), while the yield of pentenal increases under conditions of decreasing vibrational energy content (high pressures, etc.). The fact that fluorescence stabilization does not occur in cyclopentanone can be related to the results of photolysis.