Ketene quenches the phosphorescence of both biacetyl and hexafluoroacetone. Although the quenching of hexafluoroacetone emission is nearly collision-controlled, that of biacetyl is much less efficient. The large difference in the rate constants is attributed to endothermic energy transfer from triplet biacetyl to ketene and exothermic energy transfer from triplet HFA to ketene. From the endothermicity, the lowest triplet state of ketene is estimated to be at 61 ± 1 kcal/mol above its ground state. No sensitized photodecomposition of ketene occurs as a result of energy transfer from either biacetyl or hexafluoroacetone.