The tribochemical response of hot-pressed boride ceramics, ZrB 2, B 4C, ZrB 2 + B 4 C and ZrB 2 + B 4 C + SiC, has been investigated in air and in de-ionized water. In dry air, the coefficients of friction for all materials tested were about 0.9–0.95, while in humid air of relative humidity (r.h.) 95% they ranged from 0.2–0.3, except for that of ZrB 2, which was 0.4. Hertzian fractures were noted on the worn surfaces of monolithic boride and ZrB 2 + B 4 C composite, but not on those of SiC-toughened boride or SiC-toughened ZrB 2 + B 4 C. In water, ZrB 2 underwent very severe tribochemical degradation, as suggested by the following reaction: ZrB 2+10 H 2 O → Zr( OH) 4 + 2 H 3 BO 3 + 5 H 2, while B 4C, ZrB 2 + B 4 C and ZrB 2 + B 4 C + SiC underwent less tribochemical degradation, and showed lower specific wear rates than ZrB 2 in water.