Thermodynamic properties of three oxides of niobium have been measured using solid state electrochemical cells incorporating yttria-doped thoria (YDT) as the electrolyte in the temperature range T = (1000 to 1300) K. The standard Gibbs energies of formation of NbO, NbO(2), and NbO(2.422) from the elements can be expressed as: Delta(f)G(NbO)(o) +/- 547/J . mol(-1) = -414 986 + 86.861(T/K) Delta(f)G(NbO2)(o) +/- 548/J . mol(-1) = -779 864 + 164.438(T/K) Delta(f)G(NbO2.422)(o) +/- 775/J . mol(-1) = -911 045 + 197.932(T/K) The results are discussed in comparison with thermodynamic data reported in the literature. The new results refine data for NbO and NbO(2) presented in standard data compilations. There are no data in thermodynamic compilations for NbO(2.422) (Nb(12)O(29)). In the absence of the heat capacity and enthalpy of formation measurements, only the Gibbs energy of formation of NbO(2.422) can be assessed. The free energy of formation of stoichiometric Nb(2)O(5) is evaluated on the basis of measurements on NbO(2.422) and information available in the literature on phase boundary compositions and isothermal variation of nonstoichiometric parameter with oxygen potential for Nb(2)O(5-x). The results suggest a minor revision of data for Nb(2)O(5). A minimum in the Gibbs energy of mixing for the system Nb-O occurs in the nonstoichiometric domain of Nb(2)O(5-x) with x = 0.036.