<p>The influence of the addition of purified polyhalite (K₂MgCa₂(SO₄)₄·2H₂O) at different concentrations on the phase transitions and thermal stability of the ammonium nitrate (AN) has been studied and discussed. Differential thermal analysis coupled with thermogravimetry and differential scanning calorimetry was used to evaluate the stability of the analyzed system. Analyses under both free and limited mass transfer conditions with the environment were performed. Appropriate amounts of ammonium nitrate and purified polyhalite were mixed to obtain the desired percentage of additive (5, 10, 20&#xa0;mass%). It was concluded that the purified polyhalite had a neutral or inhibitory effect on the thermal decomposition of ammonium nitrate. The effect of polyhalite on the decomposition of ammonium nitrate was highly dependent on the additive content and the conditions under which the measurements were carried out. Thermal analysis proved to be an effective tool in the preliminary assessment of the effect of raw material on the thermal decomposition of AN. Purified polyhalite was considered to be a suitable additive to ammonium nitrate and could be used in the production of multicomponent fertilizers.</p>