The photocycloadditions of p-quinones with quadricyclane (Q) and norbornadiene (N) to give oxolanes I and oxetanes II were investigated by measurements of chemically induced dynamic nuclear polarization (CIDNP) and NMR analysis of the products. With systems such as 1,4-benzoquinone/Q strong S-T-0-type CIDNP was observed. The polarizations of the starting materials and of the rearranged hydrocarbon N are exclusively generated in radical ion pairs (RIPs) whereas the product polarizations predominantly stem from biradicals III. Both cycloadducts are formed by geminate processes, I resulting from the singlet exit channel of III, and II from the triplet exit channel. III was identified as a 1,5-biradical possessing the same structure as the oxolane I but with the C-C bond between the quinone and the norbornene moiety broken. A 1,2-shift of a vinyl group converts III into a 1,4-biradical IV also comprising a norbornenyl substructure, which is the direct precursor to II. This rearrangement occurs via a further 1,5-biradical V with a nortricyclyl skeleton. Neither IV nor V give rise to CIDNP, which is explained by a smaller distance between the radical centers and a shorter lifetime compared to III. The reversible rearrangement III reversible arrow IV does not depend on the electron spin multiplicity; in combination with the ability of IV to act as a chemical sink this provides the analogue to an escape reaction of radical pair CIDNP. By the competition of this "escape" process with nuclear-spin selective intersystem crossing S-T-o-type CIDNP is generated in biradical III. Such a spin-sorting mechanism has been observed for the first time in this work. Superimposed on the biradical polarizations in the cycloadducts there are also polarizations stemming from RIPs. The relative amount of these two contributions strongly depends on the sensitizer and the quencher. From an analysis of these results and thermodynamic considerations it was inferred that electron transfer quenching precedes biradical formation whenever this is energetically feasible.