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Environmental context Knowledge of the speciation of metals, especially of the free metal ion concentration, is essential to understand the fate of these elements in rivers and their effects on living organisms. On-site analyses are preferred for these measurements as they allow problems associated with sample transportation and preservation to be avoided. In this context, an on-site methodology based on an electrochemical method and screen-printed sensors has been developed in the laboratory and validated on site. Abstract An on-site methodology has been developed for the direct determination of free Zn2+ with AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) in freshwaters. This implementation includes: (i) the use of screen-printed electrodes, which provide good limits of detection and easy transportation and deployment; (ii) no need for sample purging; (iii) a calibration in a synthetic river solution that reproduces the speciation changes of the natural samples well and allows oxygen interference to be minimised; (iv) the addition of a background electrolyte up to 0.01molL-1 in both the calibration and freshwater samples; (v) chemical stripping chronopotentiometry as the quantification stage of AGNES. This procedure minimises the effects of working at low ionic strength and in the presence of dissolved oxygen. In the laboratory, the methodology was checked with different natural samples taken from rivers Garonne, Gave de Cauterets and Gave de Pau in the Pyrenees. Results appeared in good agreement with theoretical estimations computed from Visual Minteq. On-site measurements were performed for the first time with AGNES in the Gave de Cauterets in Soulom (France) and the results were corroborated with purged measurements performed in the laboratory.