The errors that may arise from the use of the external random field (ERF) approximation in the spin relaxation theory are estimated for the case of a binary solution. The experiments by M. Ishiwata and Y. Ishii (J. Phys. Soc. Jpn. 60 (1991) 1379, ibid. 60 (1991) 1743) have been analyzed. Coupled spin-lattice relaxation of solute molecules has been measured in the experiments where the solvent spin system was exposed to the effect of nonselective pulses. Going beyond the ERF model we assume that the behavior of a solvent spin system can be characterized by a single polarization. This approach involves the consideration of coupled solvent-solute relaxation (CSSR). It has been found that the parameters of molecular motion, extracted with the CSSR approach, differ noticeably from those obtained within the scope of the ERF model. We conclude that if the solvent spin system is perturbed during the experiment and its spin-lattice relaxation time is not too short, the CSSR approach should be recommended for quantitative study of the motion of solute molecules.