The first application of Li-7 pulsed-gradient spin-echo (PGSE) diffusion methods to structural lithium chemistry is reported. The data, which provide quantitative diffusion constants at 155 K, lead to a new method of estimating solvent viscosity at this temperature and clearly show a solvent dependence for the structure of LiPPh2, In THF, LiPPh2 exists as a mononuclear solvated species, whereas in Et2O, a dinuclear structure is found, D values for the model compound PHPh2 in THF have been measured.