Abstract The A-site-deficient perovskite solid solution (ADPESS) La 0.67− x Li 3 x TiO 3 ( x=0.11) underwent an order–disorder transition reversibly with a reversible change in ionic conductivity. The cubic, disordered form ( a= a p ) was obtained by quenching from temperatures above ∼1150°C, while the tetragonal, ordered form ( a= a p , c≈2 a p ), by annealing at temperatures below ∼1150°C. Since controlled thermodynamically, the extent of ordering of La ions could be changed reversibly by annealing in a range of 600–1150°C. The equilibrated order parameter S increased continuously from 0 at 1150°C to ∼0.83 at 600°C with decreasing annealing temperature. The bulk ionic conductivity at 25°C had a small maximum at S≈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≈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.