Abstract: Para-cyano,-carbomethoxy, and-acyl substituents decrease the triplet reactivity of valerophenone (7-hydrogen abstraction), whereas comparable meta substituents increase reactivity. Spectroscopic results are presented which indicate that para-(-R) substituents lower a,** triplet energies so much more than n, a* energies that the lowest triplets become largely T, T* in nature. Meta-(-R) substituents do not stabilize a, a* triplets enough to invert ...