Oxidation of ZrN ceramics from 973-1373K under static conditions reveals parabolic rate behavior, indicative of a diffusion-controlled process. In-situ high temperature powder XRD found the oxidation mechanism begins with destabilization of ZrN through formation of a ZrN1-x phase with oxide peaks initially detected at around 773K. The zirconium oxide layer was found to be monoclinic by in-situ XRD with no evidence of tetragonal or cubic polymorphs present to 1023K. Bulk ceramic samples oxidized at 1173 and 1273K underwent slower oxidation than those oxidized at 973 and 1073K. This change in oxidation rate and hence mechanism was due to formation of a denser c-ZrO2 polymorph stabilized by nitrogen defects. This N-doped dense ZrO2 layer acts as a diffusion barrier to oxygen diffusion. However, at an oxidation temperature of 1373K this layer is no longer protective due to increased diffusion through it resulting in grain boundary oxidation.