The deviation from stoichiometry, delta, in (ZnxMn1 - xFe2)(1 -delta/3)O-4 was measured thermogravimetrically at 1200 degreesC as a function of oxygen activity, a(O2), and cationic composition, x. Point defect thermodynamic modeling of the oxygen activity dependence of delta suggests that cation vacancies are the majority ionic defects at high oxygen activity and cation interstitials dominate at lower activities. (C) 2003 Elsevier Science B.V. All rights reserved.