Photolysis of 1,5-cyclodecanedione, both in the crystalline state and in solution, fails to lead to any Norrish/Yang type II products despite the fact that the conformation of the molecule in the solid state is nearly ideal for hydrogen atom abstraction. Based on X-ray crystal structure and molecular modeling studies, it is argued that gamma-hydrogen abstraction does occur in the solid state, but that closure of the resulting 1,4-biradical is slower than reverse hydrogen atom transfer owing to difficulty in forming the expected 8-membered ring-containing photoproduct. In solution, on the other hand, molecular mechanics calculations indicate that the lack of photoreactivity of 1,5-cyclodecanedione stems from the fact that this molecule exists nearly exclusively in a conformation for which gamma-hydrogen abstraction is sterically impossible.