Synthesis was carried out by dissolving MnO, ZnO, and Fe2O 3 in the molar ratio 0.42:0.44:1.07 in fluxes of Na 2O:xB2O3 at 900°C and slow cooling under ambients of CO-CO2 mixtures; x varied from 1.34 to 2.0 and Co-Co 2 from 0.06 to 1.0% CO. The composition and oxidation state of the precipitated ferrite were dependent on both CO content and x. The composition of the ferrite ranged between Mn0.44Zn0.40Fe 2.16O4 and Mn0.09Zn0.06Fe 2.85O4 and the Fe2+ between 0.16 and 0.85 moles/formula unit. By using reactive gas mixtures, the cation oxidation states were maintained constant during cooling. Mössbauer spectroscopy and coulometric analysis were used to establish that Mn3+ was unstable in the flux in the presence of Fe2+. The results obtained by these experiments establish the optimum conditions necessary to grow Mn-Zn ferrite films by liquid phase epitaxy (LPE). The saturation magnetization of the synthesized material was between 4700 and 5200 G; the Curie temperature increased with Fe2+ from 180 to 490°C. These properties agree with measurements previously made on LPE films of this material.