The characteristics of the electrogenerated chemiluminescence in dimethylformamide of pyrene, 1,2-benzpyrene, 3,4-benzpyrene and dibenzo[ a, lpyrene, dibenzo[ a, i]pyrene, dibenzo [ a, h]pyrene and dibenzo [ b, l]pyrene generated with optimum conditions of applied potential are described. The appearance of emission at wavelengths longer than those expected from the excited singlet state of the hydrocarbons may be attributed to decomposition products. Pyrene and 3,4-benzpyrene exhibit long wavelength emission, however, which it is suggested may be due to excimer formation. Several groups of workers have studied the electrogenerated chemiluminescence emission (e.c.l.) from aromatic hydrocarbons in non-aqueous media 1–13. The emission may result from radical cation-anion annihilation between the cathodic reduction product R − and the anodic oxidation product R + to form an excited singlet state molecule R * by the reaction ▪ This appears to be the most common type of mechanism involved in ECL emission when the energy available from the reaction is sufficient to form the excited singlet state and the free radicals R + and R − are relatively stable. The emission observed usually resembles the normal fluorescence emission spectrum of the compound in the same solvent. The e.c.l. emission spectra of several hydrocarbons have been reported to show also broad emission peaks at longer wavelengths than this normal emission from the singlet 14. These have been attributed to the formation of excimers 5,14 or to emission from decomposition products of the hydrocarbons 15,16. Excimers are dimeric species which arise from the interaction of a molecule in the excited singlet state with a ground state molecule ▪ The excimer may then decompose with emission of radiation to form two ground state molecules ▪ and the emitted radiation is characteristically board and structureless. Chandross et al. 14 postulated that the reaction R + + R − → R * 2 might occur directly during the e.c.l. process to produce the similar broad emission peaks that they observed for a number of compounds. Other workers have commented on the possibility that this emission might be attributed to decomposition products rather than excited dimeric species 15,16. The assignment of the long wavelength e.c.l. emission from 9,10-dimethylanthracene to excimer formation has been made by Parker and Short 5. The e.c.l. emission of pyrene has been reported by Bard et al. 17 to be too weak to measure except when an oxidant such as tetramethylphenylenediamine is present. In a previous paper we have described apparatus which provides for the sensitive detection of e.c.l. emission from aromatic hydrocarbons in non-aqueous media and reported the emission characteristics of a range of such compounds. We observed intense emission from pyrene in dimethylformamide with tetrabutylammonium perchlorate electrolyte. This communication describes a study of the e.c.l. of pyrene and a series of related fused ring hydrocarbons. The purpose of this work has been two-fold, ( a) to assess the suitability of the measurement of the e.c.l. emission of these compounds for their detection and determination, and ( b) to establish whether excimer species similar to those reported for pyrene derivatives in solution fluorescence by Birks and Christopherou 18 are formed during the e.c.l. process.