The thermal diffusivities of ZrB2–SiC (10.7, 21.9, or 48.7 vol% SiC) with B4C sintering aid were measured over 25–2000 °C using laser flash. The composition with the highest SiC showed the highest thermal conductivity (k) at 25 °C, but the lowest above ≈400 °C, because of the greater k temperature sensitivity of the SiC phase. Finite difference calculations of k, using selected literature data for the individual phases, and the concentration of phases from microstructures, correctly predicted temperature and phase concentration dependencies, but were lower than experimental results. The k of pure ZrB2 and SiC as a function of temperature were back-calculated from the experimental results for the multi-phase materials; they were in good agreement with specific literature values.