The single crystal of nickel complex of 3-nitro-1,2,4-triazol-5-one (NTO), [Ni(H2O)(6)](NTO)(2).2H(2)O was prepared and the crystal structure was determined by a four-circle X-ray diffractometer. Based on the results of thermal analysis, the thermal decomposition mechanism of [Ni(H2O)(6)](NTO)(2).2H(2)O was derived. From measurements of the enthalpy of solution in water of [Ni(H2O)(6)](NTO)(2).2H(2)O at 298.15 K, the standard enthalpy of formation, lattice enthalpy and lattice energy have been determined as -(2585.6 +/- 4.5), -2832.59, and -2797.58 kJ mol(-1), respectively. (C) 2003 Elsevier B.V. All rights reserved.