A method of applying a descaling reagent comprising a one-electron reducing agent which is a low oxidation state transition metal ion in combination with a complexing agent to a surface to be treated to descale the surface which method comprises:(i) maintaining a low oxidation state transition metal ion either in solution under an inert atmosphere in a container made of or lined with an inert material or as a solid salt under an inert atmosphere;(ii) preparing a solution of the complexing agent and removing oxygen therefrom; and(iii) mixing the ingredients from steps (i) and (ii) either in situ in contact with the surface to be treated, or mixing the ingredients from steps (i) and (ii) prior to application to the surface to be treated under conditions whereby no substantial decomposition of the so-formed reagent occurs.