In this work, we report a calculation model elaborated for the evaluation of the surface excess of radiolabeled species via measurement of the intensity of low-energy X-rays, and present some new findings obtained by the radiotracer and voltammetric studies of the Zn2+ electrosorption on polycrystalline gold. The preliminary investigations of the underpotential deposition (upd) of Zn2+ ions demonstrates the extremely high sensitivity of the radiotracer technique based on the detection of the Cu-K-1,K-2 X-radiation and assures the reliability of the model calculation procedure. The experimental results reveal that: (i) the upd of Zn2+ ions on polycrystalline gold is measurable even from solution containing 5x10(-8) mol dm(-3) Zn2+ over the entire potential range where the discharge of protons or water molecules, i.e; hydrogen evolution takes place; (ii) the fractional coverage of the surface obtained under non-equilibrium (transient) conditions in the upd region (from 0.05 to -0.35 V) is no more than 1.5%.