Abstract— Ultra‐violet irradiation of oxygen‐free solutions of 9‐methyI anthracene leads to the formation of both stable dimers and metastable excimers. The fluorescence spectra of concentrated solutions show an excimer emission band. This behaviour is attributed to the partial steric hindrance of dimer formation by the meso‐sub stituent, and it is proposed that the stable dimers and excimers correspond to anti‐parallel and parallel configurations of the two monomers respectively. A consideration of the monomer fluorescence data for anthracene and its meso‐substituted derivatives suggests that steric hindrance similarly reduces the interactions with solvent molecules which lead to internal quenching.