Heat capacity of (NH4)(2)Mg(SO4)(2)center dot 6H(2)O (CAS: 7785-18-4) was measured by low-temperature vacuum adiabatic calorimetry (AC) in the temperature range from 8 to 320 K. A linear combination of Einstein functions was applied to approximate obtained data on the heat capacity. Heat content and entropy of (NH4)(2)Mg(SO4)(2)center dot 6H(2)O in the temperature range from 0 to 320 K were calculated from these data. The molar enthalpy of (NH4)(2)Mg(SO4)(2)center dot 6H(2)O dissolution in water was measured at 298.15 K by solution calorimetry (SC). Standard entropy, enthalpy of formation and Gibbs energy formation of (NH4)(2)Mg (SO4)(2)center dot 6H(2)O at 298.15 K were calculated from these results and literature data. Melting point of (NH4)(2)Mg(SO4)(2)center dot 6H(2)O was evaluated at external pressure 10(4) kPa by differential scanning calorimetry (DSC). The reversible phase transition of (NH4)(2)Mg(SO4)(2)center dot 6H(2)O crystal aggregates was recorded using three independent methods: DSC, AC and dielectric permittivity measurements. It was shown that this transition is absent for (NH4)(2)Mg(SO4)(2)center dot 6H(2)O having smaller crystal size obtained by grinding of the crystal aggregates in an agate mortar. (C) 2017 Elsevier Ltd.