The accumulation of scale in an aqueous system is inhibited by the addition of a copolymer of maleic acid or anhydride or fumaric acid and allyl sulfonic acid and a water soluble organic phosphonic acid such as aminotri(methylene phosphonic acid), ethylenediaminetetra(methylene phosphonic acid), hydroxyethylidene diphosphonic acid and 2-phosphonobutane tricarboxylic acid-1,2,4 or water soluble salts thereof. Scale can actually be removed by the addition of these materials to pressurized boiler water systems. Allyl or vinyl copolymers, a phosphonate, and optionally a water soluble, hardness precipitating agent such as a carbonate or a phosphate may be used when treating pressurized boilers to provide an in-service cleaning effect.