We synthesized the cubic A-site deficient perovskites Sr(0.5+x)Li(0.5-2x)square(x)MO(3) (M=Ti0.5Ta0.5) and La-doped Sr0.55-1.5xLax Li(0.4)square(0.5x+0.05)MO(3) (SR55LA), Sr(0.575-1.5x)La(x)Li(0.35)square(0.5x+0.075)MO(3) (SR575LA), and Sr(0.6-1.5x)La(x)Li(0.3)square(0.5x+0.1)MO(3) (SR60LA), and their lithium ion conductivities were elucidated. Among these compounds, the highest ionic conductivity, sigma=7.2 x 10(-4) S cm(-1) at 294 K, was found in SR575LA (x=0.05). In Sr(0.5+x)Li(0.5-2x)square(x)Ti(0.5)Ta(0.5)O(3), the lattice constants were independent of the composition, and the composition dependence of the ionic conductivity indicates that the conduction mechanism involves the vacancy diffusion process of the lithium ions and vacancies that percolate through the A-site sublattice including the skeletal ions, Sr2+ as obstacles. In contrast, in La-doped systems, the composition dependence of the ionic conductivity was found to be associated with (1) the vacancy diffusion process including site percolation and (2) the reduction of bottleneck size due to the substitution of La that depresses the conductivity. Furthermore, it was found that the activation energy derived from dc conductivity is influenced by long-range cooperative ionic motion together with the local structure, e.g., bottleneck size. (C) 2004 Elsevier B.V. All rights reserved.