A procedure to determine the vacancy concentration [V] in magnetite from TG curves when heating alpha-Fe2O3 in a reducing atmosphere is presented, together with some experimental results. The procedure correlates vacancy and cation concentration with the weight change due to oxygen evolution through the charge and mass balance in the sample. The results are presented together with a generalized summary of previous findings on mixed spinel ferrites and yttrium iron garnet. It is shown that in all cases the procedure leads to the same general expression theta = Gamma mu(B)/mu(o2) (1 - m(f)/m(i)) where m(f) and m(i) are the final and initial mass of the sample, mu(B) the formula mass of the initial oxide reagents, mu(o2) the oxygen molar mass and theta a parameter associated with the oxygen evolution. The parameter Gamma, together with the explicit dependence [V](theta) (or interstitial concentration l(theta) in the case of YIG) must be determined in every case from the specific chemical equations. The Gamma values derived for different ferrite materials are tabulated together with the corresponding [V](theta) or [I](theta). The procedure allows the calculation of the Fe3+-->Fe2+ enthalpy from the equilibrium In(K) versus 1/T curves deduced from the TG data and the Van't Hoff equation.