The thermophysical and structural properties of calcium and barium zirconium phosphate ceramics (CZP and BZP) have been investigated for their potential candidacy as actinide hosts for inert matrix fuels (IMF) in nuclear reactors. These phosphate ceramics, which can accommodate minor actinides as well as the resulting fission products, are found to be thermally stable to 1600 °C in air, however they begin to decompose in an inert atmosphere above approximately 1400 °C. The heat capacity, thermal conductivity and bulk thermal-expansion were measured from room temperature up to 1200 °C. Structural changes in this temperature region as well as the anisotropic thermal-expansion behaviour were studied using high-temperature X-ray diffraction. A phase change from R-3 to R-3c was identified for Ba0.5Zr2(PO4)3 near 880 °C. The thermal conductivity for these ceramics at 1000 °C was found to be 1.0 W m-1 K-1, a relatively low thermal conductivity that was increased to 5.0 W m-1 K-1 at 1000 °C for BZP:Ni (25:75 mass ratio) cermet composites.