Toluene-de solutions of cis- and trans-cyclooctene (cis- and trans-1a) as well as (Z)- and (E)1 1-methylcyclooctene (cis-and trans-1b) have been irradiated at temperatures between -95 and + 110 degreesC in the presence of benzophenone (BP) to afford mixtures of the cis- and trans-configured oxetanes 2a,b and the regioisomeric 2b'. Correspondingly, benzoquinone (BQ) gave with cis- and trans-1a the cycloadducts cis- and trans-3a. The cis/trans diastereomeric ratios of the [2 + 2]-cycloadducts 2 and 3 display a strong temperature dependence; with cis- and trans-1a or cis-1b as starting materials, the diastereoselectivity of the oxetane formation is high at low temperature, under preservation of the initial cyclooctene configuration. With increasing temperature, the cis diastereoselectivity decreases continuously for the cis-cyclooctenes; in the case of the cis-1a, the diastereoselectivity is even switched to trans (cis/trans ca. 20:80) at very high temperatures. For the strained trans-1a, the trans-oxetanes are strongly preferred over the entire temperature range, with only minor leakage (up to 10%) to the cis-oxetanes at very high temperatures. Oxetane formation is accompanied by nonthermal trans-to-cis isomerization of the cyclooctene. The methyl-substituted trans-1 b constitutes an exceptional substrate; it displays cis diastereoselectivity in the [2 + 2] photocycloaddition at low temperatures for both regioisomers 2b and 2b', and the trans selectivity increases at moderate temperature (cis/trans = 4:96), to decrease again at high temperature, especially for the minor regioisomer 2b'. This complex temperature behavior of the cis/trans diastereoselectivity may be rationalized in terms of the triplet-diradical mechanism of the Paterno-Buchi reaction. We propose that the cyclooctene may be competitively attacked by the triplet-excited ketone from the higher (syn) or the less (anti) substituted side; such syn and anti trajectories have hitherto not been considered. To account for the unusual temperature behavior in the diastereoselectivity of the present [2 + 2] photocycloaddition, we suggest that temperature-dependent conformational changes of the resulting triplet preoxetane diradicals compete with their cyclization to the cis(trans-oxetane diastereomers and retro cleavage to the cis-cyclooctene.