  A statistical numerical analysis of thermogravimetric measurements
(temperature, mass variation, oxygen pressure) leads to a new equilibrium phase
diagram T-P-X of the iron monoxide or w\"ustite FeOx. Three varieties or pseudo
phases W1, W2, W3 are revealed above 911 {\textdegree}C, three others W'1, W'2,
W'3 below. This diagram displays their subfields, and shows 25 invariant points
between stable phases and 8 possible invariant points between metastable
phases. A thermodynamic approach is pointed out, which makes possible to
characterize the point defects and their clusters as components of the crystal
lattice.
