Arsenic is a redox-sensitive element of environmental relevance and often enriched in iron sulphides. Because sediments from the Achterwasser lagoon, a part of the estuarine system of the river Oder, south-west Baltic Sea, show unexpectedly high pyrite concentrations of up to 7·5 wt% they were used to investigate the influence of authigenic pyrite on the mobility and burial of As in the coastal environment. Micro-X-ray-fluorescence measurements of 106 micrometre-sized pyrite framboids from the anoxic sediments show highly variable As concentrations ranging from 6 to 1142 μg g−1. Even within a 1 cm thick layer, the As concentration of different framboids varies greatly and no clear depth trend is visible throughout the 50 cm long sediment core. Pyrite can account for 9 to 55% (average 22%) of the total As budget of the sediments and the degree of trace metalloid pyritization for As ranges from 26 to 61%, indicating that authigenic pyrite formation is an important process in the geochemical cycling of As in coastal sediments. High-resolution micro-X-ray fluorescence mapping of single pyrite grains shows that As is distributed inhomogeneously within larger framboids, suggesting changing pore water composition during pyrite growth. X-ray absorption near edge structure spectra indicate that As is usually present as As(-I) substituting S in the pyrite lattice. However, in samples close to the sediment/water interface a considerable part of As is in higher valence states (+III/+V). This can be explained by frequent re-suspension of the surficial sediments to the oxic water column due to wave action and subsequent re-deposition, leading to the adsorption of As oxyanions onto pyrite. Although reduced As(-I) becomes more important in the deeper samples, reflecting decreasing redox potential and a longer time since deposition, the occurrence of oxidized As species (AsIII/AsV) in pyrite in the anoxic part of the sediment suggests formation under dysoxic conditions.