Abstract The phase relations of iron vanadium sulfides with the atomic Fe: V ratios 3 : 2 and 71 : 29 were examined by the high-temperature DTA and X-ray measurements. It was found that the structure of iron vanadium sulfide changes gradually from a less symmetric form to a highly symmetric form with the increase in temperature. Two types of phase transformations by vacancy order-disorder were observed at temperatures above 800 °C. One was due to the intralayer disordering of metal vacancies: V3S4-type→Cd(OH)2-type (type Ia) or V3S4-type+NiAs-type→Cd(OH)2-type (type Ib) at T1. The other was due to the entirely interlayer disordering of metal vacancies: Cd(OH)2-type→NiAs-type (type II) at T2 (T2>T1). The tentative phase diagram of iron vanadium sulfides with the atomic Fe: V ratio 3 : 2 was constructed on the basis of the DTA results.