The nature and extent of oxygen removal from Nb–C–O alloys on heating to 2273K under 2mPa vacuum has been investigated. Correlations have been given relating the vapour pressure ratios of CO, NbO(v) and NbO2(v) to the residual oxygen and carbon contents in niobium and the temperature of treatment. Experimental results of treating Nb pellets with initial oxygen content of 1.09wt% and added carbon corresponding to C/O ratios of 0–1.10 in a thermogravimetry unit up to 2273K under 2mPa have been explained on the basis of the relevant vapour pressure ratios. The deoxidation of Nb–C–O begins at 1650K and proceeds in three distinct steps consisting of both sacrificial deoxidation and carbon deoxidation. The first step is sacrificial deoxidation due to formation of NbO2(v), the second step is carbon deoxidation, and the final step is sacrificial deoxidation by NbO(v) vaporization. The extent of deoxidation in each of these steps depends on C/O ratio of the charge.