The triplet energy accepting properties of phenylethene (styrene), 2-methyl-1-phenylpropene and phenyladamantylidene have been examined. It is shown that the degree of non-vertical behaviour, i.e. the donor triplet energy range over which non-classical behaviour is observed, increases with the degree of torsion about the formal single bond between two adjacent components of a conjugated π system. The result strongly supports the contention that the key to non-vertical behaviour, as it applies to conjugated π systems, is torsion about formal single bonds on the ground state surface.