According to the impedance model of the electrode process on a heterogeneous metal surface, two parallel processes are taking place at two different parts of the surface characterized by different energies and therefore, occur independently from each other. Two separate impedance models can be developed for these processes. In the present work the diagnostic coordinates for four equivalent electrical circuits are developed in which experimental impedance spectra are straight lines. The proposed diagnostic coordinates allow fast and reliable evaluation of the experimental data for the processes occurring on the heterogeneous metal surfaces. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.007205esl] All rights reserved.