The stability and structure of aqueous complexes formed by pentavalent antimony (SbV) with simple organic ligands (acetic, adipic, oxalic, citric acids, catechol and xylitol) having O-functional groups (carboxyl, alcoholic hydroxyl, aliphatic and aromatic hydroxyl) typical of natural organic matter (NOM), were determined at 25°C from potentiometric and X-rayabsorption fine structure spectroscopy (XAFS) measurements. In organic-free aqueous solutions, spectroscopic data are consistent with the dominant formation of SbV hydroxide species, Sb(OH)5 and Sb(OH)6-, at acid and near-neutral to basic pH, respectively. Potentiometric measurements demonstrate negligible complexing with mono-functional organic ligands (acetic) or those having non-adjacent carboxylic groups (adipic). In contrast, in the presence of poly-functional carboxylic, hydroxyl carboxylic acids and aliphatic and phenolic hydroxyl, SbV forms stable 1:1 or 1:3 complexes in coordination 6 with the studied organic ligands, over a wide pH range pertinent to natural waters (3 = pH = 9). The XAFS measurements show that in these species the central SbVatom has an octahedral geometry with 6 oxygen atoms from hydroxyl moieties and adjacent functional groups (O = C–OH and/or C–OH) of the ligand, forming bidendate chelate cycles. Stability constants for SbV-oxalate complexes generated from potentiometric experiments were used to model SbV complexing with di-carboxylic functional groups of natural humic acids. Our predictions show that in an aqueous solution of pH between 1 and 4 containing 1 ¿g/l of Sb and 5 mg/l of dissolved organic carbon (DOC), up to 15% of total dissolved Sb maybe bound to aqueous organic matter via di-carboxylic groups.