Carbon was used to reduce oxides in spark plasma sintered ZrB2 ultra-high temperature ceramics. A thermodynamic model was used to evaluate the reducing reactions to remove B2O3 and ZrO2 from the powder. Powder oxygen content was measured and carbon additions of 0.5 and 0.75 wt% were used. A C-ZrO2 pseudo binary diagram, ZrO2-B2O3-C pseudo ternaries, and Zr-C-O potential phase diagrams were generated to show how the reactions can be related to an open system experiment in the tube furnace. Scanning transmission electron microscopy identified impurity phases composed of amorphous Zr-B-O with lamellar BN and a Zr-C-O ternary model was calculated under SPS sintering conditions at 1900 degrees C and 6 Pa to understand how oxides can be retained in the microstructure.