In this work we report modelling of electrochemical impedance spectra for the CO bulk electrooxidation on Pt, an electrochemical system with an S-shaped current-potential curve (S-NDR (negative differential resistance) system). Galvanostatic control and parameter ranges, in which self-organized stationary CO-coverage patterns exist on the electrode surface are considered. The patterns consist of two stationary domains with different CO coverages. The simulations show that at very low frequencies the applied current modulation imposes a domain size modulation which occurs at nearly constant electrode potential. Consequently, the interface impedance modulus tends to zero at very low frequencies and no negative real impedance can be observed in the simulated impedance spectra. For higher modulation frequencies, the faster current modulation affects the domain size expansion and contraction processes, inducing an increase of the interface impedance modulus and a dependence of its phase on the frequency. These results demonstrate that the dynamics of pattern formation affects considerably the linear response of an electrochemical system. Furthermore, they suggest that measurements of impedance spectra can offer valuable information on the dynamics of pattern formation and of the electrochemical processes involved. (C) 2009 Elsevier Ltd. All rights reserved.