In this study, both the deviation from stoichiometry delta and the cation tracer diffusion coefficients, D-Me*, of cobalt, iron and manganese ions have been measured as a function of oxygen activity, a(O2), for different cationic compositions, x, in (CoxMn1-x)(3-delta)O-4 at 1200 degrees C. All delta versus log a(O2) plots obtained are in principle S-shaped. The observed oxygen activity dependence suggests that cation vacancies are the predominant point defects at high a(O2) while cation interstitials prevail at low a(O2). From the diffusion results it follows that all log D-Me*, versus log a(O2) curves are V-shaped. This behavior indicates that cation vacancies and cation interstitials govern the high temperature cation diffusion at high and low oxygen activities, respectively. By combining nonstoichiometry and diffusion data mean mobilities of cations diffusing via different point defects have been determined. At high oxygen activities the prevalent cation diffusion mechanism is a vacancy diffusion process. The diffusion mechanism prevailing at low oxygen activities is very likely an interstitialcy diffusion process.