The pressure-temperature-composition equilibrium is studied in the metal-rich portion of the niobium-nitrogen system. In the terminal solid solution, Sieverts' Law is found to be obeyed for all compositions studied. ΔHN, the heat of solution of nitrogen in niobium is given as −46 kcal per g-atom of nitrogen; ΔSN°, the entropy of solution at 1 atom per cent solute is given as −14.7 E.U. per g-atom of nitrogen, in good agreement with the theoretical configurational entropy of solution. An empirical expression for the equilibrium concentration of nitrogen in solution is given as x = 6.2 × 10−4(pN2)12exp(46,000RT) where x is atom per cent nitrogen and pN2 is in mm of Hg. Nb2N is found to be the nitride in equilibrium with the solid solution. The standard enthalpy and entropy of formation of Nb2N above 1500°C are indicated as −67 kcal per g-atom of nitrogen and −22.3 E.U. per g-atom of nitrogen respectively. The terminal solubility of nitrogen in niobium with respect to Nb2N is measured from 300°C to 2230°C and found, with one exception, to be in general agreement with earlier studies. An expression for the terminal solubility above 1150°C is x = 720 exp (−20,000RT). It is concluded that nitrogen exhibits nearly ideal behavior as a solute in niobium.