Natural organic matter can change As speciation via redox reactions and complexation influencing its mobility and toxicity. Here we show that binary and ternary colloids and dissolved complexes of As(V), Fe and organic matter (OM) form at environmentally relevant conditions and analyzed these colloids/complexes using ATR-FTIR- and Mossbauer-spectroscopy. Dissolved Fe-OM complexes and ferrihydrite-OM colloids were formed by reacting OM with ferrihydrite (Fe(OH)(3)). Mossbauer-spectroscopy showed that 95% of the Fe in the Fe-OM fraction were present as ferrihydrite-OM colloids while the remaining 5% were in the dissolved fraction. In As(V) plus Fe-OM systems (containing both dissolved and colloidal Fe-OM), 3.5-8 microg As(V)/mg OC was bound to the Fe-OM complexes/colloids compared to <0.015 microg As(V)/mg OC in As-OM systems (without Fe). Upon filtration of As-Fe-OM complexes/colloids with a 3 kDa filter, approximately 6% As was found in the dissolved fraction and approximately 94% As in colloidal Fe-OM. This suggests that As(V) is associated with Fe-OM mainly via ferrihydrite-OM colloids but to a small extent also in dissolved Fe-OM complexes via Fe-bridging. Since As-contaminated soils and aquifers contain Fe(III) minerals and OM, colloids of As with OM-loaded ferrihydrite and complexes of As with dissolved Fe-OM have to be considered when studying As transport.