In aqueous HClO4 solutions the protonated colloidal SnO2 particles undergo a two-stage reduction at renewed mercury electrodes, the second stage being discernible at voltammetric curves. First are reduced the protons on the surface of the colloid; the hydrogen atoms thus formed reduce subsequently SnO2 to metallic tin. At more negative potentials then prevails a direct reduction of hydroxo complexes of tin on the surface of the particles to metal. A side product of both reactions, an aggregate of SnO2 colloid deprived of its stabilizing positive charge, adsorbs at the electrode surface and hinders further electroreduction.