A variable-thickness, variable-temperature absorption cell, modified from the design of Holden, Taylor, and Johnston, has been used to compare the infrared spectrum of diethyl ketone at −75° and +25 °C. The Raman spectrum of diethyl ketone has also been measured at +6°, +25°, and +85 °C, and the proton magnetic resonance spectrum over the range −61° to 104 °C.Both the infrared and Raman spectra exhibit pronounced temperature sensitivity, which is attributed to the presence of at least two conformational isomers in the liquid phase. Possible structures for these isomers are considered. Small but significant chemical shifts are noted in the proton magnetic resonance spectrum.