Abstract The stability field of solid wüstite has been the subject of many thermodynamic and structural investigations. These reveal three varieties - W 1, W 2 and W 3 - above 911°C and three other varieties - W′ 1, W′ 2 and W′ 3 - below this temperature. A statistical analysis of our measurements, and of those found in recent literature, of the lattice parameter of wüstite as a function of composition and temperature confirms the existence of the three high-temperature varieties. A dilatometric study leads to the same conclusion. A similar analysis could be made for the three other varieties before studying their structure, but their existence has already been directly evidenced by thermogravimetric and structural investigations.