High pressures of cis-butene-2 interact with an excited state, postulated to be the singlet, of hexafluoroacetone in such a way as to inhibit the decomposition of the ketone but to yield only small, and perhaps negligible, amounts of isomerization (part III). Studies of the undispersed emission support the hypothesis that the singlet state is involved; when present at high pressures both isobutene and cis-butene-2 reduce the fluorescence yield to zero. The fluorescence spectrum is decreased in intensity, but essentially unchanged in shape, in the presence of moderate amounts of biacetyl. Quantitative measurements of both these effects were made and values for the rate constants for the quenching of the singlet state were derived. An approximate value of the quenching constant obtained from work on the decomposition in the presence of cis-butene-2 agreed as to order of magnitude with that derived from the emission study.