Manganese ferrite (MnFe₂O₄) nanoparticles were synthesized through a coprecipitation method using manganese (II) chloride tetrahydrate (MnCl₂·4H₂O) and ferric chloride hexahydrate (FeCl₃·6H₂O) as precursors. The scanning electron microscopy images showed that the as-synthesized particles were granular and about 20 nm. The X-ray diffraction patterns revealed that the manganese ferrite phase was completely decomposed into ferric oxide (Fe₂O₃) and manganese (III) oxide (Mn₂O₃) after annealing above 800°C in air. In contrast, its crystalline quality significantly improved when it was annealed in argon. By using the vibrating-sample magnetometry technique, it was demonstrated that the saturation magnetization (M _s) of the as-prepared sample (~36.6 emu/g) decreased sharply up to ~5 emu/g after annealing at 1000°C in air and significantly increased to ~77.6 emu/g when it was annealed at 1000°C in argon. Under sunlight radiation, a higher efficiency was observed for manganese ferrite particles annealed in argon in the presence of hydrogen peroxide (H₂O₂), mainly due to the Fenton reaction between manganese ferrite and hydrogen peroxide. The authors suggest that the presence of hydrogen peroxide and the enhancement of the crystalline quality of the manganese ferrite phase are the two leading factors in improving methylene blue degradation efficiency.