The thermal decomposition process of solid NbCl4 was studied by a tensimetric method using an ordinary and a two-zone membrane manometer. It was shown that the solid NbCl4 decomposed to the upper limit of a solid solution on the basis of a niobium trichloride crystal lattice and gaseous NbCl5. The homogeneity region of this solid solution is NbCl3.13-2.67 and it decomposes from an upper to a lower border with formation of the same gaseous product. After complete decomposition of the solid solution to its lower limit in the closed system the NbCl2.67 (cluster Nb3Cl8 form reacts with gaseous NbCl5 with formation of gaseous NbCl4. The thermodynamic characteristics of solid NbCl3.13 and NbCl2.67 and gaseous NbCl4 were calculated on the basis of experimental data:Delta(f)H degrees(NbCl3.13, s, 298.15 K) = -599 +/- 10 kJ mol(-1) . Delta(f)H degrees(NbCl2.67, s, 298.15 K)= -536.4 +/- 10.2 kJ mol(-1) Delta(f)H degrees(NbCl4 g, 298.15 K) = -577.1 +/- 8.5 kJ mol(-1) . Delta(s)H degrees(NbCl4, 298.15 K)= 128.4 +/- 7.1 kJ mol(-1).