Levels established in water quality guidelines for the protection of aquatic life are based on the total concentrations of heavy metals regardless of speciation. However, there are numerous studies that show the importance of determining both the physicochemical characterisation of water body and the total concentration of heavy metals in them, including its chemical speciation, given its specific correlation with the bioavailability. In this regard, the objective of this study is to quantify concentrations of heavy metals in Los Patos lagoon, Argentina, and to show the utility of estimating with Visual MINTEQ software the fractions in which these metals are forming aqueous inorganic species or complexed with organic matter. The results demonstrate the relevance of using speciation calculations of metal cations, particularly when their concentrations are in the order of guide levels.Centro de Investigaciones del Medioambiente