The Li mass transport mechanisms of the NaTl-type intermetallic compounds beta -LiAl and beta -LiIn were studied using the Pulsed Field Gradient (PFG) NMR spin echo technique over a temperature range of 297-485 K. The lithium self-diffusion coefficients in beta -LiAl alloys follow Arrhenius behavior. The Li diffusion coefficients and the associated diffusion constant D//o(Li) and activation energy LT AN BR E RT AN BR decrease with increasing Li content. A vacancy diffusion mechanism based on two mean jump times for vacancies yields a quantitative description of the Li diffusion process under a V//L//i-Li//A//l attractive interaction. High values of Li diffusivities (10** minus **6cm**2/sec at room temperature) which increase with decreasing Li content were observed in beta -LiIn alloys. Over the temperature range 300-400 K, the Li diffusion coefficient obeys the Arrhenius relation. The diffusion constant D//o(Li) and activation energy LT AN BR E RT AN BR decrease with increasing Li content. The results suggest that there is a repulsive interaction between the vacancy (V//L//i) and the antistructure atom (Li//I//n). Theoretical analysis indicate that this V//L//i-Li//I//n repulsive interaction is so strong that it goes into the second nearest neighbor distance.