Marine ferromanganese oxides contain a large amount of trace elements, such as arsenic (As) and molybdenum (Mo). However, the host phases of tetrahedral AsO4 3− and MoO4 2− oxyanions therein have not been clearly identified thus far. In this work, we explored the mineralogical components of hydrogenetic (HG) ferromanganese nodules and compared the distribution behaviors of As and Mo. The X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) analyses showed that the predominant manganese and iron phases were vernadite (δMnO2) and ferrihydrite, respectively. According to the sequential extraction examination, both As and Mo were associated with the iron (oxyhydr)oxide phases. However, the XAS analyses indicated that As was retained by the ferrihydrite phase via double cornershared complexation, while Mo was preferentially adsorbed on δ-MnO2 via edge-shared complexation. The immobilization of As and Mo by HG ferromanganese samples was attributed to specific chemical binding (ΔGchem) rather than Coulombic interaction (ΔGcoul) as proposed in previous studies. The comparison of Mo XAS spectra before and after extraction revealed the unreliability of the sequential extraction approach to determine the host phase of trace elements as a result of the potential readsorption risk. The different distribution trends of As and Mo were due to their disparate intrinsic properties (e.g., averaged dissociation constants of conjugate acids) and the diverse properties (i.e., surface site densify, adsorption equilibrium constant, and crystalline structure) of ferrihydrite and δ-MnO2. These research findings would be significant for evaluating the geochemical behaviors and environmental fate of trace elements in marine systems.