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2,3-Diphenylnorbornene and cis-stilbene exhi bit essentially identical 'nonvertical' triplet excitation behaviour, in agreement with a mechanism requiring torsion about formal single bonds on the ground-state surface.The 'nonvertical' or non-classical behaviour of certain flexible n-systems with respect to endothermic triplet energy transfer has been a subject of considerable mechanistic discussion1 which has centred around the cis-stilbene molecule. In this communication we report that 2,3-diphenylnorbornene 1 , a molecule with essentially the same n-system geometry as cis-stilbene, but with a central double bond which cannot undergo significant torsion on either the ground state or triplet state surfaces, exhibits virtually identical 'nonvertical' triplet excitation behaviour. This is a clear indication that the 'nonvertical' phenomenon, as it relates to flexible n-systems, reflects single-bond as opposed to double-bond torsion. The