Unusual electrochemical immittance spectra (eis) recorded at Cu rotating disc electrode in CUSO4 + H2SO4, solution and analogous to spectra at states close to oscillations reported by other authors, were interpreted as representative for non-minimum phase (nmp) and minimum phase (mp) systems. The electrode was stable under voltage control (potentiostatic conditions, pc) but its stability under current control (galvanostatic conditions, gc) changed at Hopf bifurcation corresponding to zero admittance (infinite impedance). CNLS fitting procedure was applied using 3rd order rational function of complex frequency and from its parameters the electrical equivalent circuit and zero-pole representations of electrode impedance were calculated. All zeros were real negative numbers (stability under pc); two, out of three, poles were complex conjugate numbers, which in case of dataset II (stability under gc, mp behaviour) had negative real parts and for dataset I (unstable under gc, nmp behaviour)-positive real parts. Transition from nmp- to mp-type of immittance resulting from short anodic polarization was attributed to local activation of the passive electrode corresponding to the appearance of parallel conductance. This was demonstrated by numerical simulation showing transformation of the mp-type immittance to the nmp-type one as a result of increasing parallel conductance. (C) 2004 Elsevier Ltd. All rights reserved.