The phosphorescence of chloropentafluoroacetone at 77 °K is reported and the energy of the triplet state is estimated to be 74 kcal. The phosphorescence lifetimes and the relative phosphorescence yields of chloropentafluoroacetone and hexafluoroacetone at 77 °K were measured. The results show that substitution of a chlorine atom in the fluorinated aliphatic ketone changes the radiative and non-radiative rate constants from the triplet state at 77 °K by factors of about 1.7 and 2.3, respectively.