Careful measurements of the T₁-S₀ origin band frequencies and spin-sublevel spacings of the lowest triplet states of ten isotopically labeled benzophenones, isolated as guest molecules in DDE host crystals at 1.2 K, show that these properties are extremely sensitive to H-2, C-13, O-17, and O-18 substitution. These changes are interpreted here to identify the principal routes (and mechanisms) by which the photoexcited molecule in its T₁ state returns to the ground state. These are spin-orbit coupling with the ground state and vibronic coupling with the T₂ state.