Iridium electrodes were modified by adsorption of sulphur species produced by reaction with H2S, and tested for the electrocatalytic reactions of oxidation and cathodic evolution of hydrogen. At low sulphur coverages (theta(s) <0.4-0.5) the current densities for the hydrogen evolution and oxidation reactions decrease slightly (at low overpotentials) or increase (at larger overpotentials), resulting in a maximum for theta(s)=0.20. At higher sulphur coverages (theta(s) >0.50) a stronger decrease of current densities is seen. These effects of sulphur obtained in the case of iridium electrodes, are quite comparable with those obtained in the case of platinum electrodes. The Tafel slope increases with increasing sulphur coverage from 35 mV, for theta(s)=0, to 60 mV, for theta(s)=0.95, indicating a change of mechanism at high theta(s), whereas in the case of platinum electrodes this parameter remains unchanged in the presence of adsorbed sulphur.