Processing parameters of a two-step carbothermal reduction–nitridation of ZrO2 as a UO2/PuO2 simulant to fabricate ZrN and ZrCxNy were examined. Variables studied were reaction time, reaction temperature and starting powder composition. Higher temperatures, longer reaction times and higher initial amounts of carbon in the starting material lead to increased nitridation similar to previous work with uranium oxide but reaction rate decreases with time. SEM and TEM of the reacted powders reveal regions of ZrC with ledges indicative of crystal defects with small particles of ZrN growing on the ZrC bulk material suggesting a reaction that occurs via Zr transport to the reaction site either through mass diffusion or evaporation–condensation of Zr(g) and with N2(g) forming the nitride. Annealing experiments on these mixed phases show formation of a solid solution which is rapid compared to the nitridation, where there is little to no formation of a solid solution.