A transition from a ductile to brittle behaviour has been observed in tosylate-doped polypyrrole resulting from reduction and oxidation of the polymer. The electrochemical potential at which the transition occurred was found to depend upon the electrolyte used, with sodium nitrate and sodium chloride undergoing reduction at higher potentials than occurred with magnesium nitrate and magnesium chloride. The ductile -brittle transition observed was found to be due to the formation of ionic crosslinking during oxidation, and their removal during reduction. The plasticization expected from the concomitant incorporation and expulsion of ionic species (with considerable water of hydration) was found to have a smaller effect. © 1998 Elsevier Science S.A. All rights reserved.