The irradiation of 2-aryl-4-( -3'-aryl-allylidene)-5(4 )-oxazolones with blue light (456 nm) in the presence of Ru(bpy) (BF ) (bpy = 2,2'-bipyridine, 5% mol) gives the unstable cyclobutane-bis(oxazolones) by 2+2-photocycloaddition of two oxazolones . Each oxazolone contributes to the formation of with a different C=C bond, one of them reacting through the exocyclic C=C bond, while the other does so through the styryl group. Treatment of unstable cyclobutanes with NaOMe/MeOH produces the oxazolone ring opening reaction, affording stable styryl-cyclobutane bis(amino acids) . The reaction starts with formation of the T excited state of the photosensitizer Ru*(bpy) , which reacts with S of oxazolones through energy transfer to give the oxazolone T state (oxa*)- , which is the reactive species and was characterized by transient absorption spectroscopy. Measurement of the half-life of (oxa*)- for , and shows large values for and (10-12 μs), while that of is shorter (726 ns). Density functional theory (DFT) modeling displays strong structural differences in the T states of the three oxazolones. Moreover, study of the spin density of T state (oxa*)- provides clues to understanding the different reactivity of 4-allylidene-oxazolones described here with respect to the previously reported 4-arylidene-oxazolones.