The single crystal of cobalt compound of 3-nitro-1,2,4-triazol-5-one (NTO) ([Co(H2O)(6)](NTO)(2).2H(2)O) was prepared and the crystal structure has been determined by a four-circle X-ray diffractometer. The compound is monoclinic with space group Cc and unit cell parameters of a = 2.3163 (2) nm, b = 0.6451 (1) nm, c = 1.4123 (1) nm, beta = 123.96 (1)degrees, V = 1.7504 (3) nm(3) and Z = 4. Based on the thermal analysis, the thermal decomposition mechanism of [Co(H2O)(6)](NTO)(2).2H(2)O has been derived. From measurements of the enthalpy of solution in water at 298.15 K, the standard enthalpy of formation, lattice enthalpy and lattice energy have been determined as -(2594.7 +/- 4.5), -2753.19, and -2718.19 kJ mol(-1), respectively. (C) 2003 Elsevier B.V. All rights reserved.