Thermal conductivities of Cs-M-O (M = Mo or U) ternary compounds, observed in the pellet-cladding gap region and in the pellet periphery in irradiated oxide fuels with high oxygen potentials, were investigated. Bulk samples of Cs2MoO4 and Cs2UO4 were prepared by hot pressing or spark plasma sintering, and their thermal diffusivities were measured by the laser flash method from room temperature to 823 K for Cs2MoO4 and to 900 K for Cs2UO4. The thermal conductivities were evaluated from the thermal diffusivity and bulk density, and the specific heat capacity values available in the literature. The thermal conductivities of Cs2MoO4 and Cs2UO4 were quite low compared with UO2 (e.g. 0.5 Wm−1K−1 at 800 K for Cs2MoO4).