9-Anthroylacetone undergoes a head-to-tail 4π+4π photo-dimerisation reaction that leads to the formation of 5,11-bis(1,3-diketobutyl)-5,6,11,12-tetrahydro-5,12,6,11-di- o-benzeno-dibenzo a, ecyclooctene both in solution and in the solid state when irradiated with different sources (sunlight, tungsten lamp, xenon lamp, UV laser beam 351–364 nm), the reaction being accompanied by a colour variation from bright yellow to colourless. Quantum yields >0.023 mol/Einstein are evaluated for the solid state reaction. Interestingly, the dimer dissociates to give 9-anthroylacetone, both thermally ( T>130 °C) and photochemically, by short UV wavelength irradiation. The single-crystal X-ray structure of 9-anthroylacetone and its dimer are reported. 9-Anthroylacetone undergoes a head-to-tail 4π+4π photo-dimerisation reaction both in solution and in the solid state when irradiated with different sources; the dimer reversibly dissociates into 9-anthroylacetone, both thermally and photochemically.