Unusual electrochemical immittance spectra (eis) recorded at Cu rotating disc electrode in CuSO4+H2SO4 solution are analogous to spectra at states close to instability and oscillations reported recently by other authors. In the literature they are often referred to as spectra with negative resistance (nr). They can be also looked at as typical for non-minimum phase (nmp) systems representing states stable under voltage control (potentiostatic conditions-pc) and unstable under current control (galvanostatic conditions-gc). Kramers-Kronig (KK) transformation tests for such eis data are valid when done on admittance representation but fail when done on impedance data representation. (c) 2005 Elsevier B.V. All rights reserved.