Polypyrrole films modified with Ni Particles were eletrodeposited using a constant potential on a Pt/quartz crystal electrode with the aim to investigate the changes in the voltammetric and mass profiles compared with pure polypyrrole. The electrochemical behavior of the polypyrrole with Ni showed a new cathodic peak at -0.73 V associated to Ni2+ reduction. Besides, it was characterized that there is a redox-pair of the polymer and Ni and a model was proposed to explain the phenomenon considering the eletroneutrality principle. The transport of the ClO4-ion into (or outside) the polymer is opposite to the expected direction. This occurs due to the redox-pair between the PPy chain and Ni species which can be explained by a loss of anions during Ni oxidation and the gain of anions during the Ni reduction. (C) 2005 Elsevier B.V. All rights reserved.