We describe experimental results and theoretical models for nuclear and electron spin relaxation processes occurring during the evolution of F-19-labeled geminate radical pairs on a nanosecond time scale. In magnetic fields of over 10 T, electron-nucleus dipolar cross-relaxation and longitudinal Delta HFC-Delta g (hyperfine coupling anisotropy -g-tensor anisotropy) cross-correlation are shown to be negligibly slow. The dominant relaxation process is transverse Delta HFC-Delta g cross-correlation, which is shown to lead to an inversion in the geminate F-19 chemically induced dynamic nuclear polarization (CIDNP) phase for sufficiently large rotational correlation times. This inversion has recently been observed experimentally and used as a probe of local mobility in partially denatured proteins (Khan, F.; et al. J. Am. Chem. Soc. 2006, 128, 10729-10737). The essential feature of the spin dynamics model employed here is the use of the complete spin state space and the complete relaxation superoperator. On the basis of the results reported, we recommend this approach for reliable treatment of magnetokinetic systems in which relaxation effects are important.