Abstract Mg2+ ion-exchange with the Li+ ion in a fast Li ion-conducting perovskite La0.55Li0.35TiO3 was successful using MgCl2–KCl molten salt. The lattice parameter decreased as a result of the ion-exchange. These results suggest that the driving force of the ion-exchange reaction depends primarily on a decrease in the lattice energy, accompanied by the substitution of the smaller Mg2+ for the larger Li+ ion. A polycrystalline ion-exchanged sample was found to show considerable Mg2+-ion conductivity (4.1×10−6 S cm−1) at a lower temperature, 285 °C, than the synthesis temperature, 450 °C.