[Mg(H2O)(6)](NTO)(2).2H(2)O was prepared by adding magnesium carbonate hydroxide to the aqueous solution of 3-nitro-1,2,4triazol-5-one (NTO). The single crystal structure has been determined by a four-circle X-ray diffractometer. The compound is monoclinic with space group C2/c and unit-cell parameters a = 2.3101(3) nm, b = 0.6472(l) nrn, c = 1.4118(3) nm and beta = 124.05(1)degrees. From measurements of the enthalpy of solution in water at 298.15 K, the standard molax enthalpy of formation, lattice energy and lattice enthalpy have been determined as -(3015.38+/-4.2) kJ mol(-1), -2680.7 and -2653.2 kJ mol(-1).