Two SiC-containing metal diborides materials, classified in the ultra-high-temperature ceramics (UHTCs) group, were fabricated by hot-pressing. SiC, sinterability apart, promoted resistance to oxidation of the diboride matrices. Both the compositions, oxidized in air at 1450 degrees C for 1200 min, had mass gains lower than 5 mg/cm(2). Slight deviations from parabolic oxidation kinetics were seen. The resistance to thermal shock (TSR) was studied through the method of the retained flexure strength after water quenching (20 degrees C of bath temperature). Experimental data showed that the (ZrB2+HfB2)-SiC and the ZrB2-SiC materials retained more than 70% of their initial mean flexure strength for thermal quenchs not exceeding 475 degrees and 385 degrees C, respectively. Certain key TSR properties (i.e., fracture strength and toughness, elastic modulus, and thermal expansion coefficient) are very similar for the two compositions. The observed superior critical thermal shock of the (ZrB2+HfB2)-SiC composite was explained in terms of more favorable heat transfer parameters conditions that induce less severe thermal gradients across the specimens of small dimensions (i.e., bars 25 mm x 2.5 mm x 2 mm) during the quench down in water. The experimental TSRs are expected to approach the calculated R values (196 degrees and 218 degrees C for ZrB2+HfB2-SiC and ZrB2-SiC, respectively) as the specimen size increases.