Method of removing metals from hydrocarbon feedstock using esters of carboxylic acids, and additives for the same, are provided, wherein hydrocarbon stream such as crude oil containing metals and slats thereof, such as calcium and calcium naphthenate, is mixed with an effective metal - removing - amount of an aqueous extraction - solution of non - precipitating and non - fouling additive comprising a chemical compound selected from a group consisting of methyl or ethyl or propyl or isopropyl mono - and / or di-esters of any of three carboxylic acids, such as, maleic acid, maleic anhydride, or fumaric acid or an appropriate combination of said esters, or an appropriate combination of any of said esters with any of said three acids, enabling formation of a hydrocarbonous phase and an aqueous phase containing the metal ions; and separating aqueous phase.