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Phosphorescence decay studios on aromatic hydro-carbons dissolved in rigid matrices have been carried out. Phosphorescence lifetimes of deuterated hydrocarbons are given. The effects of the presence of oxygen and of changes in temperature and solvent on the phosphorescence life-times and intensities have been investigated. It is shown that oxygen has a considerable effect in reducing the lifetimes and intensities, even when the hydrocarbons are dissolved in rigid matrices. The role of rigid media in allowing phosphorescence to be observed is discussed and it is concluded that the rigidity or degree of polymerisation does not affect first order phosphorescence decay; rather it is the permeability of the medium to oxygen and other impurities that is of importance. Glasses of low permeability have been investigated and shown to be efficient media for the study of phosphorescence. There is some evidence to show that phosphorescence is temperature dependent. Activation energies for internal conversion from the lowest triplet to the ground state have been determined; these arc found to be much smaller than those earlier reported. Energy transfer between triplet states has been studied. A slow transfer mechanism, not previously recorded, has bean demonstrated. The nature of this transfer is discussed and a mechanism proposed.