Abstract Nanocrystalline MnFe 2 O 4 particles were synthesized by a high-energy ball milling technique, starting from a manganosite (MnO) and hematite (α-Fe 2 O 3 ) stoichiometric powder mixture. The mechanosynthesis process was performed at room temperature both in hardened steel and in tungsten carbide vials. X-ray powder diffraction quantitative phase analysis by the Rietveld method was used to study the chemical transformations promoted by the milling action. The nanocrystalline MnFe 2 O 4 spinel phase begins to appear after 10 h of milling and reaches its maximum content (≈0.8 molar fraction) after 35 h of milling. A prolonged milling time induces a dramatic contamination of the powder mixture, when hardened stainless steel was adopted, due to metallic iron originating from vial and balls debris. Ball milling is able to induce a redox reaction between Fe III and metallic iron, transforming the MnFe 2 O 4 spinel phase into a wustite type (Fe, Mn)O phase. The yield of the hydrogen production reaction on synthetised materials is reported.