The deviation from stoichiometry, delta, in (CrxFe1-x)(3-delta)O-4 was measured by thermogravimetry at 1200 degrees C as a function of oxygen activity, a(02), and cationic composition, x. Moreover, cation tracer diffusion coefficients, D-Me*, for Me = Co, Cr, Fe and Mn, were experimentally determined using radioactive isotopes, Co-60, Cr-51, Fe-59 and Mn-54. delta versus log a(02) plots for different (x) displayed S-shaped curves. The experimental data suggested that cation vacancies and cation interstitials were the majority defects at high and low oxygen activities, respectively. V-shaped curves were observed in D-Me* versus log a(02) plots. The observed oxygen activity dependence indicated that the diffusion of Co, Cr, Fe and Mn in (CrxFe1-x)(3-delta)O-4 at high oxygen activities is governed by cation vacancies and at low oxygen activities by cation interstitials. Mean cation mobilities were derived by combining non-stoichiometry and cation diffusion information.