The A-site-deficient perovskite solid solution (ADPESS) La0.67-xLi3xTiO3 (x = 0.11) underwent an order-disorder transition reversibly with a reversible change in ionic conductivity. The cubic, disordered form (a = a,) was obtained by quenching from temperatures above similar to 1150 degrees C, while the tetragonal, ordered form (a = a(p), c approximate to 2a(p)), by annealing at temperatures below similar to 1150 degrees C. Since controlled thermodynamically, the extent of ordering of La ions could be changed reversibly by annealing in a range of 600-1150 degrees C. The equilibrated order parameter S increased continuously from 0 at 1150 degrees C to similar to 0.83 at 600 degrees C with decreasing annealing temperature. The bulk ionic conductivity at 25 degrees C had a small maximum at S approximate to 0.2 and decreased increasingly rapidly with increasing extent of ordering in a range of S > 0.2. The tetragonal distortion of the subcell began at S approximate to 0.2. The distortion became more manifest and the lattice parameters a contracted more with increasing extent of ordering in a range of S > 0.2. The decrease in ionic conductivity is attributed to an increase in activation energy for ionic conduction, which is presumably associated with the contraction of the lattice parameter a of the subcell.