The thermal properties were investigated for hot-pressed zirconium diboride - transition-metal boride solid solutions. The transition-metal additives included hafnium, niobium, tungsten, titanium, and yttrium. The nominal additions were equivalent to 3 at.% of each metal with respect to zirconium. Powders were hot-pressed to nearly full density at 2150⁰C using 0.5 wt% carbon as a sintering aid. Thermal diffusivity was measured using the laser flash method. Thermal conductivity was calculated from the thermal diffusivity results using temperature-dependent values for density and heat capacity. At 25⁰C, the thermal conductivity ranged from 88 to 34 W·(m·K)-1 for specimens with various additives. Electrical resistivity measurements and the Wiedemann-Franz law were used to calculate the electron contribution of the thermal conductivity and revealed that thermal conductivity was dominated by the electron contribution. The decrease in thermal conductivity correlated with a decrease in unit cell volume, indicating that lattice strain may affect both phonon and electron transport in ZrB2.