Thermal diffusion of cations has been studied in face-centered cubic Li 2 SO 4 containing small amounts of other sulfates (temp. range 590—750°C), in body-centered cubic LiAgSO 4 containing about 0.5% of other alkali sulfates (450—550°C) and in body-centered cubic LiNaSO 4 (546 to 588°C). For the fcc systems the size of the added cation was of great importance for the thermal diffusion. Thus, for the small Na + and Ag + ions, the Soret-coefficients (σ) are of the order of only 10 -4 degr. -1 . The direction has not been established with certatinty, although there are indications that Ag + , and also Ca 2+ , are enriched on the cold side, while Na + might be slightly enriched on the hot side. There is a significant enrichment of the large alkali ions (K + , Rb + , Cs + ) on the hot side; σ=— 2 × 10 -3 degr. -1 for K + . In some experiments with these large alkali ions phase boundaries were intersected, and the large ion was always enriched in the high-temperature phase. In bcc Li 2 SO 4 —Ag 2 SO 4 (containing 40 mole % Li 2 SO 4 ) the light component Li + was enriched at the hot side: σ ≦ 1.7 × 10 -3 degr. -1 . Regarding added alkali ions, Na + is enriched at the hot side (σ ∼ 2 × 10 -3 degr. -1 ), while the direction is not established with certainty for K + and Rb + . In bcc LiNaSO 4 no significant separation of the two cations could be detected, and the Soret-coeffi-cient is certainly less than 10 -3 degr. -1 . A comparison of fcc and bcc sulfates with molten salts regarding thermal diffusion, electromigration mobility of cations, and the conductivity changes caused by impurities, shows a distinction between the fcc systems on one side and bcc sulfates and melts on the other. This supports previous conclusions that the fcc systems are characterised by a higher degree of order than the bcc ones.