Tartaric acid is often used to form complexes with and stabilize dissolved Sb(III) in adsorption studies. Sb(III) mobility in natural aquatic environments is generally enhanced by dissolved organic matter. Competition between such low molecular organic acid complexation and adsorption of Sb(III) has received little attention, which prompted us to measure Sb(III) adsorption by iron oxyhydroxide adsorbents commonly used in water treatment plants. Sb K-edge X-ray absorption fine structure spectra gave Sb–O and Sb–Fe distances and coordinations compatible with a bidentate binuclear inner-sphere complex with trigonal Sb(O,OH)3 polyhedra sharing corners with Fe(O,OH)6 octahedra and a bidentate mononuclear inner-sphere complex with Sb(O,OH)4 tetrahedra at alkaline pH. Experimental batch titration data were fitted using the charge-distribution multi-site surface complexation (CD-MUSIC) model, taking competitive effects for the organic ligand into consideration. The adsorption efficiency at acid–neutral pH decreased as the Sb(III) concentration increased. The CD-MUSIC adsorption model indicates that this was solely caused by strong competition from tartrate complexation with adsorption constants up to three orders of magnitude (logK units) higher than the iron oxyhydroxide adsorption constants without taking this competition into account. The study provides consistent CD-MUSIC surface complexation stability constants, allowing the new database to be used to model reliably adsorption of toxic oxyanions in anoxic aqueous environments, in particular also to model yet difficult to understand competition between Sb(III) and As(III).