The thermodynamic stability of spinel and rock‐salt structure phases has been investigated for the quasi‐ternary mixed oxide system Co‐Fe‐Mn‐O at 1200°C and at total pressures of the order of 1 atm (∼1.01 × 105Pa) using thermogravimetric and electrical conductivity measurements. The results reveal the stability limits of the spinel and rock‐salt structure phases with varying cationic composition and oxygen partial pressure at 1200°C. The oxygen partial pressure was varied over 12 orders of magnitude, from log10ao2= 0 to log10ao2= ‐12, by using CO/CO2and N2/O2gas mixtures and monitored by an electrochemical, stabilized zirconia‐based EMF cell. The maximum cobalt mole fraction used in the investig