Arsenic mobility in natural environments is controlled primarily by adsorption onto metal oxide surfaces, and the extent of this sorption may be influenced by the presence of other dissolved substances that interact with such surfaces or with the arsenic itself. Natural organic matter (NOM), a prevalent constituent of natural waters, is highly reactive toward both metals and surfaces and is thus a clear candidate to influence arsenic mobility. The objectives of this study were therefore to reveal the influences of a group of six diverse NOM samples on the sorption of arsenic onto a metal oxide, as well as to reveal influences of arsenic on sorption of NOM, using concentrations and conditions relevant to natural freshwater environments. Several NOM samples formed aqueous complexes with arsenate and arsenite, with the extent of complexation varying with the origin and particularly with cationic metal content of the NOM sample. Consistent with this result, NOM dramatically delayed the attainment of sorption equilibrium of both arsenate and arsenite onto hematite, the model metal oxide. The reverse was not true, however: NOM sorbed onto hematite equally rapidly whether As had previously sorbed onto the surface or not. All NOM samples displaced sorbed arsenate and arsenite from hematite surfaces, and As oxyanions similarly displaced sorbed NOM from hematite, indicating that competition between NOM and As may be an important process in natural waters. Arsenite was consistently complexed to a lesser extent, mobilized to a greater extent indicating that interactions with NOM may partially explain the generally greater mobility of arsenite in natural environments. In addition, every NOM sample rapidly oxidized arsenite to arsenate, and several also reduced arsenate to arsenite, indicating that NOM may strongly influence arsenic redox speciation, providing another mechanism by which it may affect arsenic mobility in natural environments