Infrared and Raman spectra of cyclobutane photodimers of 2-cyclopentenone, 2-cyclohexenone, 3-methyl-2- cyclohexenone, and isophorone were recorded. A comparison of the vibrational data for the head-to-head and head-to-tail anti species of each of the four sets of dimers has been used, in conjunction with the Mutual Exclusion Rule, to identify the centrosymmetric dimer. © 1969, American Chemical Society. All rights reserved.