The quenching of the fluorescence of anthracene excited by light of wave-length 3650 Å by oxygen, sulphur dioxide, and by carbon tetrabromide, has been investigated both in the gaseous state and in paraffin solutions of viscosities from 0·003 to 1·9 poises. In the gaseous state quenching occurs on nearly every collision. The quenching in dilute solution is shown to be controlled ( a ) by static quenching whereby light is absorbed by anthracene molecules contiguous to quencher molecules, and ( b ) by the diffusion together of excited molecules and quencher molecules. The former process is of greater importance in solutions of high viscosity. The latter predominates at more usual viscosities, and is found to be more nearly proportional to the diffusion coefficient of anthracene than to the fluidity of the solvent. Measured diffusional quenching constants are in good agreement with calculated values. The magnitudes of static constants indicate that in solutions containing anthracene and a quenching substance the two solutes are not randomly distributed but tend to associate with one another.