The Norrish/yang type II photochemistry of ten even-numbered cyclic diketones ranging in ring size from 10-membered to 26-membered has been studied in the crystalline state as well as in solution, In the solid state, the diketones undergo stereoselective cyclobutanol formation in which the cis or trans ring fusion stereochemistry of the photoproducts is governed by the conformation of the diketone present in the crystal as determined by X-ray crystallography. The reactive gamma-hydrogen atoms are identified and the distance and angular parameters associated with their abstraction are derived from the crystallographic data. For the most part, the abstractions occur through boatlike rather than chairlike six-atom geometries, and the average value of d, the C=O ... H abstraction distance, for 16 reactive gamma-hydrogens was found to be 2.74 +/- 0.04 Angstrom; the average values of the angular parameters omega (the gamma-hydrogen out-of-plane angle), Delta (the C=O ... H-gamma angle), and theta (the C-H gamma ... O angle) are 53 +/- 5 degrees, 83 +/- 4 degrees, and 115 +/- 2 degrees, respectively.