In the article are introduced research results of physicochemical properties of ammonium nitrate – potassium nitrate water system at 25o C. The literary source analysis is performed that no clear opinion about solid phases composition. The different composition double salts, solid solution and eutectics are identifies. Appeal is specifies a look and properties of formed structures. Methods of differential thermal analysis and X-ray diffraction have been establishing that the equilibrium solid phase of the studied system represents two types of solid solutions. A first type based on the III phase of ammonium nitrate crystal latitude (at the content of nitrate of potassium to 20 %) and a second type based on the potassium nitrate rhombic syngony (at the content of nitrate of potassium more than 80 %). Crystal cells parameters of being formed phases submit to the Vegard rule. The ammonium nitrate sample (at the content of nitrate of potassium more than 12 %) keeps phase stability at the temperature range minus 50 to plus 100о С. Burst in components miscibility at the potassium nitrate concentration range 30 to 80 % is observed.

Keywords: ammonium nitrate, potassium nitrate, differential thermal analysis, X-ray diffraction, cocrystallization, solid solution