Zirconium diboride ceramics were prepared with additions of up to 50 vol.% TiB2. The resulting (Zr,Ti)B2 ceramics formed complete solid solutions based on x-ray diffraction. The addition of TiB2 resulted in grain size decreasing from 22 μm for nominally pure ZrB2 to 7 μm for ZrB2-50 vol.% TiB2. The thermal conductivity at 25°C ranged from 93 W/m⋅K for nominally pure ZrB2 to 58 W/m⋅K for ZrB2-50 vol.% TiB2. Thermal conductivity was as high as 67 W/m⋅K for nominally pure ZrB2 at 2000°C, but dropped to 59 W/m K with the addition of 50 vol.% TiB2. Electrical resistivity measurements were used to calculate the electron contribution to thermal conductivity, which was 76 W/m⋅K for nominally pure ZrB2 decreasing to 57 W/m⋅K when 50 vol.% TiB2 was added. The phonon contribution to thermal conductivity did not change significantly for ≤ 10 vol.% TiB2. Additions of ≥ 25 vol.% TiB2 reduced the phonon contribution to nearly zero for all temperatures.