Activities of hydrogen evolution reaction, HER, on two differently modified metal-free GC electrodes and on the same electrodes supplied with ruthenium catalyst, have been studied in H2SO4 electrolyte solution. GC electrodes were gradually modified by electrochemical oxidation/reduction procedure, changing morphology properties and forming spatially heterogeneous surfaces. Ruthenium was deposited on the top of two differently modified GC electrodes in nearly the same specific mass of similar to 25 mu g cm (2) of active ruthenium, showing almost uniform dispersion of ruthenium particle clusters on less modified electrode and pronounced agglomeration on more modified electrode surface. Results of cyclic voltammetry and polarization experiments, aiding mostly in adjustments of the specific masses of active ruthenium on two GC electrodes and characteristic potential regions of "double-layer" vs. HER responses, were found strongly correlated with electrochemical impedance data. Evaluations of impedance data were done using standard regression procedure based on strictly postulated statistical criteria, in conditions of complex interfacial impedance/frequency functions accounting for: a) spatial surface heterogeneity, b) diffusion controlled hydrogen absorption and c) hydrogen evolution involving hydrogen adsorption. Activities for HER on bare GC electrodes were found much lower than on the corresponding Ru/GC electrodes, but increased with stage of surface modification. At Ru/GC electrodes, HER is proceeding exclusively on ruthenium particles with activity related to the mass of active ruthenium and total ruthenium utilization of similar to 25%. Not any effect of the supporting GC electrode morphology has been observed for HER on Ru/GC electrodes. (C) 2015 Elsevier Ltd. All rights reserved.