Yttrium tetraboride was synthesized by reactions of Y2O3 with B4C or with a B4C/C mixture. Fully dense YB4 ceramics were prepared by hot-pressing at 1800 degrees C and 20 MPa in He. The flexural strength and microhardness of the YB4 were about 300 MPa and 27 GPa, respectively. Pure YB4 showed considerable oxidation above 1200 degrees C. The effect of SiC, AlN, and AlN/SiC on the properties of YB4 ceramics was characterized. No chemical interactions were identified in the YB4-SiC and YB4-AlN pseudo binaries. The addition of SiC improved the oxidation resistance below 1200 degrees C. At higher temperatures, the low viscosity of a surface melt caused its continuous runoff and the loss of the oxidation-protection capability of the scale. The addition of AlN/SiC improved the oxidation resistance compared to the ceramics containing only SiC, which was attributed to the presence of Al2O3 in the Y2O3-SiO2-B2O3 glass leading to an increase in its viscosity and preventing glass runoff. A significant improvement in the oxidation resistance of the YB4 ceramics and extension of their oxidation stability to 1400 degrees C was accomplished by the introduction of AlN only.