In acetonitrile, head-to-head: head-to-tail photodimer ratios are independent of ketone concentration, being 4:5 for cyclopentenone and 2:1 for cyclohexenone. The variation in dimer quantum yields as a function of ketone concentration indicates ki/ka ratios of 0.06 and 2.7, respectively, where ka is the bimolecular rate constant for reaction of triplet ketone with ground-state ketone and ki is the rate of unimolecular triplet decay. Stern-Volmer quenching studies with conjugated dienes yield measures of triplet lifetimes as a function of ketone concentration. These studies indicate ka values of 6.9 X 108 and 1.1 X 108 M−1[sec−1 and ki values of 4 X 107 and 3 X 108 sec−1, respectively, for cyclopentenone and cyclohexenone. For both ketones dimerization occurs from the lowest triplet; the more rapid decay rate of the reactive cyclohexenone triplet probably reflects a greater ease of t isting. The maximum efficiency of dimerization is only 36% for cyclopentenone and 74% for cyclohexenone. Since intersystem crossing is 100% efficient for both ketones, these results indicate that both dimeri zations proceed through metastable dimeric species, significant percentages of which revert to two ground-state monomers. A comparison of these results with others in the literature suggests that dimerization occurs from π,π* triplet states. © 1969, American Chemical Society. All rights reserved.