Quantum yields for the benzophenone-sensitized cis-trans photoisomerization and dimerization of cis-and trans-l, 3-pentadiene and trans, trans-and c «, c/i-2, 4-hexadienehave been measured at high 1, 3-diene con-centrations (1-10 M). Enhanced isomerization quantum yields are obtained for c/sl, 3-pentadiene and the 2, 4-hexadienes, but isomerization quantum yields decrease with increasing diene concentrationin the case of trans-1, 3-pentadiene. The results can be accounted for by assuming that interaction of 1, 3-diene triplets with ground-state diene molecules involves two competing paths:(i) triplet excitation transfer (the quantum chain process) leading to enhanced isomerization, and (ii) biradical formation leadingto dimers and diminishedisomerization. The insensitivity of tt^ ct/0tt-* cc and 0cc-> ct/<£ cc-> tt ratios to diene concentration provides strong evidence in support of the quantum chain mechanism.