Determining antimony speciation is essential to understand its environmental behaviour and potential (eco)toxicological effects. Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) is an electroanalytical technique that has been applied up to date to the determination of the free metal ion concentrations of Zn(II), Cd(II), Pb(II) and In(III). In this work, the first application of AGNES to the measurement of the concentration of one particular species of Sb(III) in aqueous solution is developed. The extensive hydrolysis of this element, even at low pH, suggests the convenience of performing calibrations and AGNES measurements in terms of the Sb(OH)3(aq) concentration, rather than the “free” ion. A new preconcentration factor, Y′, is also introduced as the ratio between [Sb0] in the amalgam and [Sb(OH)3(aq)] in the solution. Y′ dependence on pH is evaluated with calibrations at different pH values. The experimental values of Sb(OH)3(aq) obtained with AGNES in the titration of antimony with the organic ligand oxalate agree very well with the values predicted with the complexation constants recently reported. This promising methodology opens the way to tackle antimony speciation with AGNES in other systems.