The invention provides method for reducing the concentration of a contaminant in an oil. In the method a mixture of the oil, hydrogen and an aqueous solution of a basic additive is exposed to a hydrogenation catalyst at a reaction temperature between 250°C and 500°C and at a reaction pressure sufficient for at least some of said aqueous solution to exist as a partially condensed vapour at said reaction temperature to form a product mixture. The oil is then separated from the product mixture.
