The heating of B4C-YTZP (where YTZP denotes yttria-stabilized zirconia polycrystals) mixtures, under an argon atmosphere, generates B4C-ZrB2 composites, because of a low-temperature (<1500C) carbide-oxide reaction. Composites derived from mixtures that include greater than or equal to 15% YTZP are better sintered than monolithic B4C that has been fired under the same conditions. Firing to similar to 2160 degreesC (1 h dwell) generates specimens with a bulk density of greater than or equal to 91% of the theoretical density (TD) for cases where the initial mixture includes greater than or equal to 15% YTZP. Mixtures that include 30% YTZP allow a fired density of greater than or equal to 97.5% TD to be attained, The behavior of the B4C-YTZP system is similar to that of the B4C-TiO2 system. Dense B4C-ZrB2 composites attain a hardness (Vickers) of 30-33 GPa.