Lithium intercalation into the perovskite series Li0.5-xNaxLa0.5TiO3 (0 <= x <= 0.5) has been investigated. In these perovskites, the existence of vacant A-sites is caused by the unusual location of lithium at the center of square windows that connect contiguous A-sites (unit cell faces of the perovskite). The amount of lithium intercalated into this perovskite series decreases in a nonlinear way as the amount of sodium increases. In the case of the Li-end member of the series, the amount of Li inserted is 0.5 per formula; however, in the Na end member, the intercalation process does not take place. The behavior observed can be explained on the basis of the three-dimensional vacancy percolation model previously proposed for the long-range Li mobility in Li0.5-xNaxLa0.5TiO3 series. In this model, the amount of vacant A-sites participating in Li conductivity is reduced below the percolation threshold n(p)=0.3 by substituting Li by Na. The Rietveld analysis of neutron diffraction patterns of the starting and lithiated samples shows that the intercalated Li ions also occupy unit cell faces of the perovskite. However, for the lithiated sample, the existence of important positional disorder effects impedes an estimation of the Li motion contribution to Li thermal factors. (c) 2005 The Electrochemical Society.