In acetonitrile, the photoreactions of cis-anethole, cA, or trans-anethole, tA, with fumarodinitrile, FN, lead to isomerization of both substrate and quencher and to mixed [2 + 2] cycloaddition. By NMR analysis and by NOE measurements it was shown that the same four stereoisomers of 1-anisyl-2-methyl-3,4-dicyanocyclobutane are formed in equal yields regardless of whether the substrate is cA or tA. The configuration of the anethole-derived moiety in these adducts is always trans, whereas all possible configurations of the cyano groups occur, From Stern-Volmer experiments it was concluded that the quenching mechanism is electron transfer, not exciplex formation. Electron-transfer quenching is also the pathway leading to the cycloadducts, as was established by photoinduced electron-transfer sensitization. These photoreactions give rise to strong nuclear spin polarizations (CIDNP) in the starting and isomerized forms of both substrate and quencher as well as in the cycloadducts. Radical pairs A(.+)FN(.-) (RP I) consisting of the radical cation of the anethole and the radical anion of fumarodinitrile were identified as the predominant source of the polarizations.