P-31 chemically induced dynamic nuclear polarization (CIDNP) studies of the photochemistry of p-acetylbenzyl dimethyl phosphite (1) provide evidence for the operation of the relatively rare T---S intersystem crossing mechanism for a photochemically generated tripler free radical pair. The mechanism of intersystem crossing of the triplet radical pair p-acetylbenzyl (2) and dimethoxyphosphonyl (3), generated from 1, is influenced by the large P-31 hfcc of 3 and switches from the T---S mechanism in a relatively low magnetic field (18.8 kG) to the conventional T-0-S pathway at a higher field (58.8 kG). This change in mechanism is evidenced by the photolysis of 1 in the presence of radical scavengers (halocarbons, thiophenol) which yields the corresponding products from atom abstraction by 3 that has escaped the initial radical cage. These escape products are absorptively polarized at high field (T-0-S) but are emissively polarized at lower field (T---S) in a particularly well-defined display of the effect of magnetic field strength on intersystem crossing mechanism for a geminate radical pair. Photolysis of 1 in C6D6 solution yields emissively polarized dimethyl p-acetylbenzylphosphonate (4) from combination of radical pair [2,3] in the initial solvent cage and following diffusive formation of [2,3] free pairs. Cage recombination and disproportionation of secondary F pairs, comprised of 2 and the phosphorus-substituted cyclohexadienyl radical (5), affords several emissively polarized products (6, 7, and 9) via the T-0-S mechanism at both magnetic fields. The polarization of 4 remains emissive at both magnetic fields regardless of the presence of radical scavengers, a finding which is argued to be consistent with the above results.