Metal borides and carbides, TiB2/TiC and ZrB2/ZrC are widely used nanostructured composite materials. A detailed thermodynamic analysis was performed to determine the formation conditions of titanium and zirconium borides and carbides in the Ti–B–O–C and Zr–B–O–C systems. The complete thermodynamic analysis was carried out in vacuum for the reactions 2TiO2 + B2O3 + 8C = TiB2 + TiC + 7CO and 2ZrO2 + B2O3 + 8C = ZrB2 + ZrC + 7СО. On the basis of the theoretically found results, experimental synthetic routes were developed to prepare TiB2/TiC and ZrB2/ZrC composite materials.