The speciation of As in wetlands is often controlled by natural organic matter (NOM), which can form strong complexes with Fe(III). Here, we elucidated the molecular-scale interaction of arsenite (As(III)) with Fe(III)-NOM complexes under reducing conditions. We reacted peat (40-250 μm size fraction, 1.0 g Fe/kg) with 0-15 g Fe/kg at pH <2, removed nonreacted Fe, and subsequently equilibrated the Fe(III) complexes formed with 900 mg As/kg peat at pH 7.0, 8.4, and 8.8. The solid-phase speciation of Fe and As was studied by electron paramagnetic resonance (Fe) and X-ray absorption spectroscopy (As, Fe). Our results show that the majority of Fe in the peat was present as mononuclear Fe(III) species (RFe-C = 2.82-2.88 Å), probably accompanied by small Fe(III) clusters of low nuclearity (RFe-Fe = 3.25-3.46 Å) at high pH and elevated Fe contents. The amount of As(III) retained by the original peat was 161 mg As/kg, which increased by up to 250% at pH 8.8 and an Fe loading of 7.3 g/kg. With increasing Fe content of peat, As(III) increasingly formed bidentate mononuclear (RAs-Fe = 2.88-2.94 Å) and monodentate binuclear (RAs-Fe = 3.35-3.41 Å) complexes with Fe, thus yielding direct evidence of ternary complex formation. The ternary complex formation went along with a ligand exchange reaction between As(III) and hydroxylic/phenolic groups of the peat (RAs-C = 2.70-2.77 Å). Our findings thus provide spectroscopic evidence for two yet unconfirmed As(III)-NOM interaction mechanisms, which may play a vital role in the cycling of As in sub- and anoxic NOM-rich environments such as peatlands, peaty sediments, swamps, or rice paddies.