Electrical conductivity and thermoelectric power of the ferrous ferrite system of (Mg0.29-yMnyFe0.71)3-$delta$O4 have been measured as function of the thermodynamic variables, cationic composition(y), temperature(T) and oxygen partial pressure(Po2) under thermodynamic equilibrium conditions at elevated temperatures. On the basis of the electrical properties-phase stability correlation, the stability regions of the ferrite spinel and its neighboring phases have been subsequently located in the log Po2 vs. y and log Po2 vs. 1/T planes in the ranges of 0 y 0.29, 1100 T/$^{circ}C$ 1400 and 10-14 Po2/atm 1. The stability region, Δlog Po2(y, 1/T), of the ferrite spinel single phase widens with increasing Mn-content(y) and the boundaries of each region are linear against 1/T with negative slopes.