A theoretical method of calculation of the lattice parameter of titanomagnetites, based on the concept of invariance of the distance "anion-cation" is presented. For the system Fe304-Fe2Ti04, experimental results obtained for synthetic titanomagnetites agree well with the theoretical curve if it is calculated by using the Akimoto model of cation distribution.Lines of equal lattice parameter between the joints Fe3O4-Ti Fe2O4 and Fe2O3-Fe TiO3 have been drawn by assuming that only the Fe3+ located in the Octaedral sites is oxidized. It is shown that our method of calculation of the lattice parameter can be used, reciprocaly, to determine the degree of oxydation of any natural titanomagnetites, even when they contain an important percentage of "minor" elements such as Al, Mg or Cr.