It was reported in our previous work that activated alumina had been successfully applied to selective adsorption of lithium ions in molten NaNO₃ at a high temperature. In the present work, the activated alumina samples were prepared by heat treatment of commercially available alumina sol at different temperatures. Their physical properties were measured, then their adsorption uptake of lithium ions were evaluated. Amorphous alumina showed higher adsorption uptake of lithium ions than any other phases of alumina and the uptake increased in proportion to the surface area of activated alumina. The lithium ion uptake per unit surface area varied slightly with the phase of alumina, implying that the surface microstructure of each activated alumina has a different adsorptive capability for lithium ions. Since the mean pore radius of each activated alumina sample was at least forty times larger than the ionic radius of lithium, it may be concluded that there is little effect of the pore size on adsorptive capabilities for lithium ions.