The behaviour of the klebelsbergite electrode-a bilayer structure formed by prolonged anodic oxidation of antimony in H2SO4 solutions (0.5-4.5 M)-is investigated under anodic, cathodic, and open-circuit conditions. The electrode system consists of a photoactive barrier-like film at the metal side upon which a thermodynamically stable porous layer of klebelsbergite (Sb4O4(OH)2SO4) is built up. Dissolution of the barrier layer proceeds in the pores of klebelsbergite, leading to the formation of a direct interface between Sb and the H2SO4 solution at open circuit. The equilibrium potential region of the electrode is determined. A significant routine bias is needed to reduce the klebelsbergite layer.