Abstract: The influence of manganese oxide minerals (cryptomelane, hausmannite, and pyrolusite) on the formation of iron oxides was studied in the FeCl2-NH4OH system at different Mn/Fe molar ratios (0, 0.01, 0.1, and 1.0) and pHs (3.0, 4.0, 5.0, and 6.0) by X-ray powder diffraction, infrared absorption, transmission electron microscopic, and chemical analyses. In the absence of Mn minerals, lepidocrocite (γ-FeOOH) precipitated at pHs 5.0 and 6.0; however, no precipitate formed at lower pHs. All the Mn minerals studied promoted the precipitation of iron oxides and oxyhydroxides. In the presence of Mn oxides, Fe2+ was oxidized to Fe3+, which hydrolyzed and precipitated as noncrystalline and/or different crystalline iron oxides and oxyhydroxides, depending on the nature of the Mn oxides present in the system. Simultaneously, Mn2+ was detected in solution after the reaction by electron spin resonance spectroscopy. The presence of cryptomelane and hausmannite resulted in the formation of åkaganeite (β-FeOOH) and magnetite (Fe3O4), respectively. Thus, the effect of Mn oxides on the formation of Fe oxide minerals in the weathering zone merits attention.