H[Pr(NTO)(4)(H2O)(4)]. 2H(2)O was prepared by mixing the aqueous solution of lithium 3-nitro-1,2,4-triazol-5-onate and the dilute nitric acid solution of Pr6O11. The single crystal structure has been determined by a four-circle X-ray diffractometer. The crystal is triclinic, space group P (1) over bar with crystal parameters of a=1.0463(10) nm, b=1.0484(10) nm, c=1.1474(10) nm, alpha=99.10(10)degrees, beta=96.96(10)degrees, gamma=97.65(10)degrees, V=1.2186(2) nm(3), Z=2, D-c=2.088 g.cm(-3), mu=21.17 cm(-1), F(0 0 0)=540. The final R is 0.0206. Based on the thermal analysis, the thermal decomposition mechanism of H[Pr(NTO)(4)(H2O)(4)]. 2H(2)O has been derived. From measurements of the enthalpy of solution in water of H[Pr(NTO)(4)(H2O)(4)]. 2H(2)O at 298.15 K, the standard enthalpy of formation, lattice energy and lattice enthalpy have been determined as -(2884.1+/-8.9), -5443.25 and -5473.25 kJ mol(-1), respectively.