Use of a nitrate salt composition Z comprising Z1 at least one alkali metal nitrate and optionally alkaline earth metal nitrate and also Z2 at least one alkali metal nitrite and optionally alkaline earth metal nitrite in an amount of Z2 in the range from 1.1 to 15.0 mol % based on the sum of Z1 plus Z2 as heat transfer or heat storage medium in apparatuses in which these heat transfer or heat storage media are comprised at a temperature in the range from 500 to 620° C. and an oxygen partial pressure over the nitrate salt composition in the range from 0.1 to 1.0 atm, wherein the molar amount of the alkali metal nitrite and optionally alkaline earth metal nitrite for a desired temperature selected from the range indicated above and for a desired oxygen partial pressure selected from the range indicated above is calculated by means of the following formula x nitrite = K 6  ( T ) K 6  ( T ) + P O   2 where the variables have the following meanings: xnitrate is the mole fraction of nitrite in a nitrite/nitrate mixture, K6(T) is the temperature-dependent equilibrium constant of the reaction nitrate⇄nitrite+½ oxygen (NO3−⇄ NO2−+½ O2), pO2 is the oxygen partial pressure and T is the temperature of the nitrate salt composition, and the calculated value of the molar concentration of the component Z2 is optionally reduced by 40% or increased by 20% and the nitrate salt composition Z is heated to a maximum operating temperature in the range from 500 to 620° C. in the first start-up of said apparatuses.