Solid-phase As, Fe, Mn, S and Al and pore-water total As, Fe, Mn and S have been determined for two cores from the Kalix River estuary. At the same station, the depositional flux of arsenic was measured sediment traps. Fe(III)-oxides persist in the anoxic zone of the sediment and act as a carrier for arsenic down to depths of 10-15 cm. The release of arsenic into the pore water is controlled by the reduction/dissolution of these oxides. As a result, internal cycling of arsenic, i.e. upward diffusion in pore water, re-adsorption onto Fe(III)-oxides in the oxidized surface layer and reburial occurs in the uppermost 10-15 cm. At the base of the arsenic cycling zone, solid-phase arsenic maxima (160-170 μg g-1) of probable diagenetic origin occur. The quantity of arsenic diffusing back towards the oxidized surface layer (2.2 μg cm-2 yr-1 As) is close to 75% of the quantity of arsenic being deposited due to sedimentation (2.9 μg cm-2 yr-1 As). However, a mass balance for the surface layer indicates that re-adsorption of arsenic onto Fe(III)-oxides may effectively prevent diffusion of dissolved arsenic back into the overlying water column. It is concluded that the interpretation of sedimentary arsenic profiles may be severely complicated due to post-depositional migration of arsenic