The stereochemistry and secondary isotope effects of the [2 + 2] photocycloaddition of trans, trans- (7), cis,cis- (8), and cis, trans-2,4-hexadiene (9), 2,5-dimethyl-2,4-hexadiene (1), and its deuterated analogues 1-d(1), 1-d(6), and trans-1-d(3) to C-60 have been investigated. A loss of stereochemistry in the cyclobutane ring for photocycloaddition of all three 2,4-hexadiene isomers 7, 8, and 9 to C-60 was observed (the trans stereochemistry in the cyclobutane ring predominates in all cases), while the unreactive double bond retained its stereochemical integrity in the adducts. The cis double bond of 9 is 1.5 times more reactive than the trans. The [2 + 2] photocycloaddition of (E)-2,4-dimethyl-2,4-hexadiene (10) to C-60 is regiospecific, affording two diastereomeric adducts, 10a and 10b, by addition on the methyl monosubstituted terminal double bond. These results, when taken in conjunction with the small inverse intramolecular secondary isotope effect (k(H)/k(D) = 0.90 +/- 0.05) in the [2 + 2] photocycloaddition of 1-d(6) to C-60, favor the formation of an open biradical intermediate in the rate-determining step.