The irradiation of exo- and/or endo-2,4,6-triphenyl-1,3-diazabicyclo[3,1,0]hex-3-ene in methanol to give 2,4-diphenyl-1-methoxybenzylimidazoline proceeds via photochemical ring opening to an enedi-imine which undergoes subsequent intramolecular nucleophilic addition at the imine carbon atom; the solvent controls the mode of cyclization.