Dense samples of ZrB2–20 vol% SiC were successfully fabricated by spark plasma sintering without the use of sintering aids. Oxidation behavior of these samples was characterized by exposing them to 1400°, 1500°, and 1600°C in an ambient atmosphere for 150 min, and by measuring the weight gains of the sample and crucible, as well as the thickness of the oxide scale and the glassy outer layer. The effects of gravity on the viscous outer layer are shown to result in significant heterogeneity within a sample. The oxidation scales were characterized by scanning electron microscopy and transmission electron microscopy with energy dispersive spectroscopy analysis. The oxide scale was found to be composed of three layers: (1) a SiO2-rich glassy outer layer, (2) an intermediate layer of a ZrO2 matrix with interpenetrating SiO2, and (3) a layer containing a ZrO2 matrix enclosing partially oxidized ZrB2 with Si–C–B–O glass inclusions.