In its doped state, the conjugated polymer poly-3,4-ethylene-dioxythiophene (PEDT) is an exceptionally stable organic electronic conductor that can withstand long time immersion in aqueous solutions and a wide range of pH without loosing conductivity. The conductivity is indirectly influenced by the pH of the surrounding medium because of the link between pH and oxidising power of oxygen, but these changes are largely reversible. Properly prepared, PEDT doped with tosylate is also stable under conditions where high current densities, exceeding 6000 A/cm(2), are passed through the material over extended time periods. At current densities around 10000 A/cm2 an irreversible break down mechanism is initiated, resulting in a fast decrease in conductivity and colouring of PEDT to a bluish-black hue. A model for the break down mechanism is proposed. (C) 2005 Elsevier B.V. All rights reserved.