The nonstoichiometry (delta) of (MgxFe1-x)(3-delta)O-4 ferrite spinel has been measured by a coulometric titration method as a function of the thermodynamic variables-temperature (T), oxygen partial pressure (P-o2) and Mg content (x). The nonstoichiometry amounts to +/- 1% order in its extent and varies sine-hyperbolically against P-o2. The latter is explained in terms of Frenkel disorder as the majority defect type. The effect of temperature and Mg content on nonstoichiometry is discussed.