The effects of fast-neutron and electron-induced defects on the diffusion properties of lithium in silicon have been studied. Lithium-diffused diodes were irradiated and later drifted with reverse bias at a constant temperature. Results show that fast-neutron and 0.9-MeV electron induced vacancies provide sites for the precipitation of lithium. Moreover, the lithium vacancy precipitate behaves like a solute in equilibrium with ions and ionized vacancies. The analysis of the lithium vacancy precipitate in this manner is analogous to the analysis of slightly soluble salts in water. The lithium diffusion coefficient for silicon exposed to fast neutrons, NN = 1.1 to 2.7 × 1014 n/cm2, can be expressed The range of (T) in the above expression is from 300 to 410°K. In addition, the lithium diffusion coefficient for silicon exposed to 0.9-MeV electrons, NE = 5 × 1015 to 3.3 × 1016 electrons/cm2 can be represented by The range of (T) in the above expression is from 300 to 330°K. Relative radiation damage between neutron and electrons was found to be in reasonable agreement with predictions based on radiation damage theory.