The isothermal oxidation of HfC powders was carried out at temperatures of 480° to 600°C at oxygen pressure of 4, 8, and 16 kPa, using an electromicrobalance. The oxidized product was identified by X‐ray analysis, Raman spectroscopy, X‐ray photoelectron spectroscopy, and electron diffraction, and the morphology of the oxidized grains was observed by scanning electron microscopy. Oxidation proceeds by two processes: a diffusion‐controlled process operates up to about 50% oxidation and a phase‐boundary‐controlled process operates above about 50% oxidation. The activation energies for both processes are the same (197 ± 15 kJ.mol−1). The change in the oxidation process is associated with the generation of cracks on the grains, resulting from the growth or expansion stress due to the formation of monoclinic HfO2microcrystallites less than 3 nm in size. In the latter process, the thickness of the diffusion layer is kept constant, being time‐independent, which allows the process to apparently obey the phase‐boundary