Unusual arsenic enrichment in manganese oxides in ferromanganese duricrust was revealed by EMPA and μ-XRF mappings. All the arsenic occurs as oxidized state As(V) which adsorbs on Fe/Mn oxides in bidentate binuclear bridging complexes. Extremely high levels of As enrichment has been found in Mn-rich hematite micro-bands. A conceptual model for arsenic attenuation in natural ferromanganese duricrusts has been proposed.