Low-temperature heat capacities of sodium 5-nitroisophthalic acid monohydrate (C(8)O(6)NH(3)Na(2)center dot H(2)O, s) were measured by a precision automated adiabatic calorimeter over the temperature range from (78 to 398) K. The experimental values of the molar heat capacities were fitted to a polynomial equation of heat capacities (C(p,m)) with the reduced temperatures (X), [X = f(T)] by a least-squares method. The smoothed molar heat capacities and thermodynamic functions of the compound (C(8)O(6)NH(3)Na(2)center dot H(2)O, s) were calculated on the basis of the fitted polynomial. The constant volume energy of combustion of the compound at T = 298.15 K was measured by a precise rotating-bomb combustion calorimeter to be Delta(c)U = -(3969.41 +/- 1.74) kJ.mol(-1). The standard molar enthalpy of combustion of the compound was determined to be Delta(c)H(m)(0) = -(3965.07 +/- 1.74) kJ.mol(-1). The standard molar enthalpy of formation of the compound was calculated to be Delta(f)H(m)(0) (C(8)O(6)NH(3)Na(2)center dot H(2)O, s) = -(408.45 +/- 2.03) kJ.mol(-1) in accordance with Hess's law. The standard molar enthalpy of solution of the compound, Delta(sol)H(m)(0) (C(8)O(6)NH(3)Na(2)center dot H(2)O, s), has been determined as (22.133 +/- 0.111) kJ.mol(-1). The standard molar enthalpy of the hydrated anion of the compound was determined as Delta(f)H(m)(0) (C(8)O(6)NH(3)(2-), aq) = (380.11 +/- 2.04) kJ.mol(-1), from the standard molar enthalpy of solution and other auxiliary thermodynamic data through a thermochemical cycle. The thermostability of the compound was investigated by thermogravimetric/differential thermogravimetric/differential scanning calorimetry (TG-DTG-DSC).