The first-order decay of triplet states of naphthalene, anthracene, phenanthrene and their halogenated derivatives has been studied in cyclohexane and viscous paraffin solutions. The decay results partly from a bimolecular quenching process and partly from a true first-order radiationless conversion to the ground state. The rate of the quenching process is a function of the solvent, and, in anthracene derivatives, of the extent to which the solutions have been irradiated. The rates of radiationless conversion can only be measured when they exceed the rates of quenching processes. This is the case for all the anthracene derivatives in viscous paraffin, for the bromoanthracenes in cyclohexane and probably for bromophenanthrene in viscous paraffins. Bromination of anthracene is ten times more effective in enhancing conversion than chlorination at the same position and halogenation at the 9, 10 positions has four times the effect of halogenation at other positions. In the one case where it could be tested (dibromoanthracene), the rate of radiationless conversion was unchanged by a 170-fold increase of solvent viscosity.