We report measurements of the solvent proton spin-lattice relaxation rate 1/T1 in a water solution of GdCl3, pH 5.5, at 5, 15, and 25°C. The data were obtained as a function of magnetic field, from 5 Oe to 12 kOe, corresponding to proton Larmor frequencies in the range 20 kHz to 50 MHz. The data are compared with theory, and it is shown that a satisfactory fit may be obtained for a range of values of the parameters of the theory. We give the results of a least squares fit of the theory to the data for two extremes of the range, one limit corresponding to the condition that the residence lifetime τM of a proton in the first hydration shell of the Gd 3+ aquoion be comparable to the spin-lattice relaxation time T 1M of the proton on the ion, the other limit corresponding to τM≪T1M. It is shown that knowledge of the temperature dependence of 1/T1 cannot distinguish between the two limits, and indeed, that unless a value for τM is obtained from independent experiments, the ambiguous results cannot be resolved. The source of the ambiguities is discussed in some detail since it is particularly clear in the present system, but considerably less so in systems containing complexes of macromolecules with paramagnetic ions, to which the present experiment is pertinent. It is also pointed out that, while there are many references to values for τM in the published literature, no experiments have been published that report values of τM for protons on the Gd3+ ion. Copyright © 1975 American Institute of Physics.