The stability and structure of aqueous complexes formed by trivalent antimony (SbIII) with carboxylic acids (acetic, adipic, malonic, lactic, oxalic, tartaric, and citric acid), phenols (catechol), and amino acids (glycine) having O- and N-functional groups (carboxyl, alcoholic hydroxyl, phenolic hydroxyl and amine) typical of natural organic matter, were determined at 20 and 60 °C from solubility and X-ray absorption fine structure (XAFS) spectroscopy measurements. In organic-free aqueous solutions and in the presence of acetic, adipic, malonic acids and glycine, both spectroscopic and solubility data are consistent with the dominant formation of SbIII hydroxide species, , at strongly acid, acid-to-neutral and basic pH, respectively, demonstrating negligible complexing with mono-functional organic ligands (acetic) or those having non adjacent carboxylic groups (adipic, malonic). In contrast, in the presence of poly-functional carboxylic and hydroxy-carboxylic acids and catechol, SbIII forms stable 1:1 and 1:2 complexes with the studied organic ligands over a wide pH range typical of natural waters (3 < pH < 9). XAFS spectroscopy measurements show that in these species the central SbIII atom has a distorted pseudo-trigonal pyramidal geometry composed of the lone pair of 5s2 electrons of Sb and four oxygen atoms from two adjacent functional groups of the ligand (O C–OH and/or C OH), forming a five-membered bidendate chelate cycle. Stability constants for these species, generated from Sb2O3 (rhomb.) solubility experiments, were used to model Sb complexing with natural humic acids possessing the same functional groups as those investigated in this study. Our predictions show that in an aqueous solution of pH between 2 and 10, containing 1 µg/L of Sb and 5 mg/L of dissolved organic carbon (DOC), up to 35% of total dissolved Sb binds to aqueous organic matter via carboxylic and hydroxy-carboxylic groups. This amount of complexed Sb for typical natural DOC concentrations