Excimers are formed by neutralisation of dimer cations of aromatic hydrocarbons when glassy solutions in methyl cyclohexane + isopentane mixtures, in cumene, decalin or squalane are allowed to warm up after radiolysis at 77 K. Squalane was found to be a useful solvent for the larger molecules. Excimer fluorescence of biphenyl, p-terphenyl, trans-stilbene, diphenylacetylene and perfluoronaphthalene has been observed : these emissions cannot be produced by photo-excitation. Naphthalene, pyrene, anthracene, 9-methylanthracene and perylene also gave excimers; in the last three cases, the emission was at shorter wavelengths than usual. cis-Stilbene, 1,4-diphenylbutadiene, 1,1-binaphthyl, phenanthrene, chrysene, triphenylene, biphenylene, acenaphthylene and fluoranthene gave negative or doubtful results. No excimer phosphorescence was detected despite the favourable conditions, but self-quenching of the phenanthrene triplet was observed. Methods of producing excimers are compared and the role of constraint by the matrix is discussed.