For the first time high-temperature polymorphic transition of ammonium sulfate (NH4)(2)SO4 (T-pt similar to 370 degrees C) was experimentally recorded and characterized by differential scanning calorimetry. Phase transitions parameters of (NH4)(2)SO4, of ammonium bisulfate NH4HSO4, of triammonium hydrogen disulfate (NH4)(3)H(SO4)(2) and of ammonium sulfamate NH4SO3NH2 were clarified. Method of determining the phase transitions parameters for thermally unstable substances, in particular for the triammonium hydrogen disulfate (NH4)(3)H(SO4)(2), was developed and tested with the help of differential scanning calorimetry. (C) 2014 Elsevier B.V. All rights reserved.