Ground and excited state self associations are observed in solution at room temperature in the case of the 9-oxa-2,3,4＇-methoxybenzobicyclo[4.3.0]non-1 (6)-ene-7,8-dione molecule. Depending on the solute concentration, the ground state associated forms are attributed to a dimer involving interaction between only two solute molecules, or to an aggregate of higher order. The theoretical simulation, using the analytical atom-atom pair potential described below, predicts several energy minima of the dimer. The two most stable dimer＇s structures are those involving a face-to-face slipped structure with head-to-tail orientation. An excimer emission is also observed subsequently to the excitation by either the absorption monomer bands or by the absorption band of the van der Waals dimer. The two adjacent molecules forming the excimer species are readily in approximately parallel planes.