Abstract: This paper presents thermal conductivity data for highly compacted Ca-smectite, Na-smectite, illite, and palygorskite as a function of density (i.e., compaction pressure), water content, and temperature. All the clays behaved similarly: thermal conductivity increased directly with density and water content. Specifically, the thermal conductivity increased from 0.63 to 1.32 W/m·K as the dry density increased from 1.2 to 1.8 g/cm3 (for a water content of 17%). An increase of water content from 6 to 17% resulted in an increase in thermal conductivity from 0.63 to 1.22 W/m·K (for a dry density of 1.6 g/cm3). Differences from one clay to the other were less important. The thermal conductivity (in W/m·K) for constant conditions of 12% of water and a dry density of 1.6 g/cm3 were: Ca-smectite 0.80, Na-smectite 0.74, palygorskite 0.71, and illite 0.69. Heating to 188°C produced only a 10% increase in the thermal conductivity.