The [2+2] photocycloadditions of quinones to cis- and trans-anethole in acetonitrile were studied by measurements of chemically induced dynamic nuclear polarization (CIDNP) and NMR analysis of the products. These reactions proceed with excellent regioselectivity to oxetanes; formation of cyclobutanes was not observed. With benzoquinone, anthraquinone, and naphthoquinone at room temperature, the predominant product is the thermodynamically most stable stereoisomer possessing trans configuration of the anethole moiety regardless of the configuration of the starting olefin. In the case of naphthoquinone, at low temperature good yields of additional stereoisomers are obtained in which the configuration of the educt anethole is preserved. Cycloaddition is accompanied by considerable one-way cis-trans isomerization of the olefin. The results of stationary and time-resolved CIDNP experiments showed that the polarizations of the photoproducts originate from a pathway that leads from correlated radical ion pairs of trip]et multiplicity to triplet biradicals, whereas the educts are regenerated by back electron transfer of singlet pairs. By combining CIDNP detection with photoinduced electron-transfer sensitization, it was established that the pathway that gives rise to nuclear spin polarizations is also the main route to the products; a significant contribution of a direct reaction of exciplexes to biradicals is thus ruled out for the systems investigated. The product distribution and its temperature dependence is explained by conformational changes of the tripler biradicals. The biradicals are formed in conformations that are determined by Coulombic interactions between their precursors or in the transition state. Subsequent relaxation to an energetically mon favorable conformation by thermally activated rotations around single bonds competes with intersystem crossing. The initial conformations finally lead to the low-temperature photoproducts, and the relaxed conformation leads to the high-temperature oxetane as well as (by scission of the singlet biradical) to trans-anethole, i.e., to one-way isomerization. The CIDNP measurements gave an activation energy for bond rotation of 11.5 kJ/mol and lifetimes of the initial triplet biradicals of 95 and 125 ps, which are very short but consistent with the failure of experiments to trap the biradicals with oxygen. For the singlet biradical in the relaxed conformation, the ratio of ring closure to scission was found to be 1.6 at 233 K and 1.0 at 330 K.