The electroreduction of palladium oxide films formed at high positive potentials in acid electrolytes was studied by voltammetry and potential step techniques. The anodic oxide layer can be described as a duplex layer, one inner in contact with the electrode surface, which is rather anhydrous and more compact, and a second one, thicker and rather hydrous which serves as a matrix for a fast electrochemical system. The electroreduction of the matrix-embedded fast electrochemical system follows the potentiodynamic and potentiostatic behaviour expected for a diffusion controlled process in a finite space. The electroreduction of the outer oxide layer can be described in terms of an instantaneous nucleation and 3-D growth with nuclei death. Both processes produce Pd2+ soluble species which can be detected by means of a rotating ring-disk electrode. At potentials lower than 0.7 V the electroreduction of O-2 formed during the growth of the Pd oxide layer can be detected.