Method of maintaining or widening the long-term operating temperature range of a heat transfer medium and/or heat storage medium comprising a nitrate salt composition selected from the group consisting of alkali metal nitrate and alkaline earth metal nitrate and optionally alkali metal nitrite and alkaline earth metal nitrite, wherein the nitrate salt composition is brought into contact with an additive comprising the components nitric acid and/or nitrous acid and oxygen-comprising gas having an oxygen partial pressure which is equal to or greater than that in air and/or oxygen-generating compounds and optionally nitrogen oxides and/or compounds which generate further nitrogen oxide.