Antimony is an element of increasing environmental significance but one whose chemical speciation has received little attention and, until recently, one which had not been modelled in any comprehensive way. This paper reports thermodynamic data for computer speciation models of antimony in aqueous, multicomponent solutions of environmental and biological interest involving low molecular mass organic ligands. The available data have been critically evaluated and entered into a thermodynamic database. The JESS suite of computer programs has been used to develop the thermodynamically-consistent mass balance equations used for modelling purposes. No data could be found for Sb(V) complexes and only a few of the existing data for Sb(III) complexes can be considered as being reliably described.