Abstract Irradiation of 1,2-diphenylcyclobutene (1) and diethylamine in hexane with light of longer wavelengths than 300 nm affords N,N-diethyl-1,2-diphenylcyclobutanamine (2a), 1,2-diphenylcyclobutane (3), and 1,1′,2,2′-tetraphenylbicyclobutyl (4). Upon similar irradiation, 1 and morpholine afford N-(1,2-diphenylcyclobutyl)morpholine 3 and 4. A mechanism involving a charge-transfer complex is proposed on the basis of the observations that the amines quenched the olefin fluorescence at a rate close to the diffusion limit in hexane, and that when diethylamine-N-d was used as an additive the deuterium was retained in the methine groups of compounds 2a, 3, and 4.