Abstract The mineral content of mixed fertilizer produced with calcium superphosphate, ammonium sulfate and potassium chloride and also the reactions during the production process were investigated by X-ray diffraction. It was observed that remarkable reactions took place between monocalcium phosphate, calcium sulfate, ammonium sulfate and potassium chloride by the addition of water for granulation. At the beginning of drying of the wet granules these minerals almost disappeared to form ammonium chloride and crystalline solutions of ammonium and potassium syngenite and those of monoammonium and monopotassium phosphate. The syngenite thus formed decomposed partly by further drying and again formed while the product was placed in a storage pile. It seems that the caking of the product which often occurs in a storage pile is caused mainly by the crystallization of syngenite which decreases the amount of free water in the product and makes other salts crystallize at the same time.