Here we report on a mixed oxide system, γ-Fe2O3 nanoparticles doped with Mn(III), where the transition from the cubic to the more stable hexagonal α-Fe2O3 structure is suppressed. When amorphous Fe2O3 is heated at 300 °C for 3 h, ferrimagnetic γ-Fe2O3 is observed as the sole product. On the other hand, when the temperature is raised to 500 °C, one observes only antiferromagnetic α-Fe2O3 as the product. However, upon doping with 8.5 wt % Mn(III), the amorphous nanoparticles crystallized to mainly the γ-Fe2O3 matrix after heating at 500 °C for 3 h, and need to be heated to >650 °C for the complete transition to the α-Fe2O3 structure to take place.