Arsenic sorption is the primary factor that governs bioavailability and mobility of arsenic in soils. However, studies on arsenate (As(V)) sorption have been mainly limited to soil minerals, and sorption reactions in soils as a whole in the presence of dissolved organic carbon are poorly understood. Mechanisms of As(V) sorption on red soil in the presence of oxalate or humic acid (HA) were studied using sorption isotherms and X-ray absorption fine structure (XAFS) spectroscopy. Arsenate, mainly as H₂AsO₄^- in soil solution at pH 6.0, was extensively sorbed by red soil mainly through ligand exchange. Both oxalate and HA could significantly decrease As(V) sorption through competing for sorption sites, and the competition was getting stronger with increasing concentrations of oxalate and HA. The XAFS data indicated that the sorbed As(V) formed inner-sphere complexes of bidentate binuclear configurations characterized by an As-Al bond distance of about 0.317 nm and an As-Fe bond distance of about 0.328 nm in red soil. The two As(V) complexes were stable regardless of As(V) loadings varying in the range studied. Their structures were free from the influence of oxalate and HA. This study illuminated the characteristics of As(V) sorption on red soil in the presence of oxalate and HA from molecular level and the findings might be of practical importance in controlling arsenate mobility and toxicity in soils.