Chemically induced dynamic nuclear spin polarizations (CIDNP) in the Paterno-Buchi cycloadducts of benzoquinone with norbornadiene or quadricyclane stem from a triplet biradical (III)-I-3 with a norbornenyl substructure. Intersystem crossing of (III)-I-3 and combination of the radical termini afford an oxolane I. By a cyclopropylmethyl-homoallyl rearrangement, (III)-I-3 is converted into a Paterno-Buchi biradical (IV)-I-3, which after intersystem crossing yields an oxetane II. The ratio of nuclear spin polarizations in I and II strongly depends on temperature through the competition of nuclear spin relaxation in (III)-I-3 with the biradical rearrangement. Using a kinetic treatment, a closed-form expression for the polarization ratio was derived. The activation energy of the cyclopropylmethyl-homoallyl rearrangement (17 +/- 2 kJ/mol) was obtained from an evaluation of the CIDNP signals.