The present invention relates to a method to desulphurize a substrate comprising petroleum or petroleum products, comprising the steps of: mixing an ionic liquid with the substrate to form a reaction mixture; providing the reaction mixture in a cathode chamber of a membrane electrochemical reactor; providing hydroxide solution in an anode chamber of the membrane electrochemical reactor; subjecting the reaction mixture to electrochemical reduction to generate superoxide ions in the ionic liquid, wherein the superoxide ions desulphurize the substrate; and wherein said reaction mixture is sparged with oxygen during the electrochemical reduction.