The Fe-Zr binary oxide adsorbents have higher arsenic adsorptive capacities than either iron oxide or zirconium oxide alone, indicating a strong synergistic effect exists between Fe and Zr oxides. However, no generally accepted in-depth explanations have been reached on the origin of this better performance. In the present study, the component phases, the active surface sites, the structure of the adsorbed As(V) surface species, and the mechanism of the synergistic effect, were investigated and elucidated using multiple advanced experimental techniques combined with quantum chemical calculations. Goethite and lepidocrocite were identified as the main Fe oxide components while amorphous zirconium hydroxide was the main Zr oxide component, respectively. A monodentate-mononuclear complex and a bidentate-binuclear complex were revealed to be dominant on the surface, respectively, when at lower and higher initial As(V) concentrations. Density functional theory calculations indicated that As(V) preferred to bind with Zr-OH rather than Fe-OH. This was verified with the As K-edge EXAFS results and XPS observations. The synergistic effect was due to a short-range ordering state, the enlarged contents of amorphous and poorly-crystalline fractions, and increased hydroxyl surface site density. These results lead to the realization that the above properties are preferred in future adsorbent preparations.