Nowadays, the most matured thermal energy storage (TES) technology for Concentrated Solar Power (CSP) plants is the use of molten solar salts (60 wt% NaNO3 - 40 wt% KNO3), but the thermal decomposition of nitrate salts can result in changes in the composition of molten salts or potential risks of NOx emissions. Up to now, all studies on the decomposition of nitrates in TES systems focus on the effect of the effect of sodium and potassium nitrates in contact with the container material and the NOx emissions formation, but none investigate the effect or contribution of impurities in it. Moreover, although the chemistry of the nitrates decomposition is well known, there is a big uncertainty in commercial solar plants on the production rate of NOx. The only cation in nitrates from mining origin is magnesium, and since it occurs as nitrate, it does contribute to NOx formation. This paper studies the effect of such impurity in the thermal decomposition of the solar salt. The results show that the impurity Mg(NO3)(2) is the main source of NOx emissions in solar salts during the commissioning of the plant due to its thermal decomposition during the melting process. But as described this NOx production can be handled by vents and abatement systems if it is needed.