The present invention relates to a process for electrolysing an aqueous solution of hydrochloric acid to chlorine in an electrochemical cell provided with an anode compartment and a cathode compartment including at least one gas diffusion cathode. The gas diffusion cathode comprises an electrically conductive web provided on at least on side thereof with a coating of a catalyst comprising rhodium sulphide for the electro-reduction of oxygen. The novel catalyst for oxygen reduction has an improved chemical stability towards highly corrosive media and a high electrocatalytic activity in the presence of organic contaminants.