The thermal and electrical properties were studied for zirconium diboride (ZrB2) containing 0-2wt% carbon. Thermal conductivity was evaluated from 25 to 2000⁰C and electrical resistivity measured from 25 to 800⁰C and linearly extrapolated to 2000⁰C. Estimated values for the Lorenz number for each composition ranged from 2.04 x 10-8 to 2.26x10-8 WΩK-2 and were used to separate the electron (84-95%) and phonon (5-16%) contributions to thermal conductivity. The estimated Lorenz numbers were lower than the theoretical value, which indicated the electron mean free path for thermal conduction in ZrB2 was less than the mean free path for electrical conduction. Thermal conductivities ranged from 99 to 87W/mK at 25⁰C and 76 to 72W/mK at 2000⁰C. Increasing carbon content, either as a second phase or in solid solution, decreased thermal conductivity across the entire temperature range by decreasing the electron contribution to thermal transport.