The thermal properties were investigated for hot-pressed zirconium diboride containing solid solution additions of tantalum, molybdenum, rhenium, vanadium, and chromium. The nominal additions were equivalent to 3 at.% of each metal with respect to zirconium. Using 0.5wt% carbon as a sintering aid, powders were hot-pressed to near full density at 2150 degrees C. Rietveld refinement of X-ray diffraction data was used to measure lattice parameters and to ensure that the additives formed solid solutions. Thermal conductivities were calculated from measured thermal diffusivities and temperature-dependent values for density and heat capacity. Thermal conductivities at 25 degrees C ranged from 88W(m.K)(-1) for nominally pure ZrB2 down to 28W(m.K)(-1) for (Zr,Cr)B-2. Electron contributions to thermal conductivity were calculated from electrical resistivity measurements using the Wiedemann-Franz law. Decreases in phonon and electron conduction correlated with the size of the metallic additive, indicating that changes in atom size in the Zr lattice positions reduced thermal transport.