The hardness of cemented carbide (WC-Co) cutting tools can be improved by the addition of TiC or ZrC to the matrix. It is possible that an even better hardness can be obtained by utilizing phase separation of a mixed (Ti, Zr)C where the mixed (Ti, Zr)C phase is stable at high temperature, but decomposes to TiC and ZrC at lower temperatures. In the present work, the decomposition is experimentally and theoretically investigated. The mixed carbide is first formed by synthesis from oxide powders at 2200°C and aging treatments are conducted at 1380 and 1450°C. X-ray diffraction analysis show that the synthesized mixed (Ti, Zr)C will decompose to TiC and ZrC. The experimental work is supplemented by thermodynamic and first-principles calculations.